Welcome to the community!!! And congrats on your new baby Cockatiel!!!
I'm so very glad that you are getting your baby from a reputable, seemingly responsible breeder who will not let him go home with you until he is FULLY and COMPLETELY Abundance-Weaned!!! NEVER bring home a baby bird that is not completely Abundance-Weaned and not off hand-feeding formula 100%; unfortunately this is extremely common because of unscrupulous breeders who want to get rid of their babies as quickly as possible to save money on supplies and their time, as hand-feeding a baby bird is not at all easy and is a full-time job in and of itself. So you're absolutely starting out on the right foot and down the safest, most successful path possible as a new parrot owner!!!
I'm also pleased and impressed that you are doing as much research as you possibly can BEFORE you bring your baby Tiel home. You've got some time yet if your baby is not yet eating much solid food at all, so take full-advantage of the time you have left to make sure you have a nice, big cage, lots and lots of toys of different types, and you have your baby's daily-diet planned-out in-full and are ready to go...
****What "staple" food is your baby going to be weaned onto by the breeder? By 'staple" food I mean the main component of your bird's daily diet, either a commercially-sold Avian Pellet or Seed-Mix. This is extremely important to find out immediately, because it sounds like your baby hasn't yet even started to wean, so right now is the time to make this decision so your breeder can wean your baby onto whatever "staple" food you choose, because otherwise it's very difficult and takes months to years to transition your bird from whatever they were waned onto by the breeder onto something else.
***I highly suggest that if you don't know already exactly what staple-diet your breeder is weaning your baby onto, that you call them immediately and find out not only whether it's a pellet or a seed-mix, but the exact brand and type. Ideally you want your bird's daily staple-diet to be an Avian Pellet, specifically a "Natural" flavored pellet, and not one of the "Fruit-flavored" pellets that are multi-colored, because they are absolutely full of sugar, which is turned into fat and stored in their livers, and not any better than a fatty seed-mix in the end. And the same goes for any of the "Nut-flavored" pellets, they are just loaded with fat/oil. So you want to choose a "Natural" avian pellet as your bird's daily staple, the best ones being any of the Harrison's pellets, TOPS pellets, Zupreem Natural pellets, Tropican pellets, In-Tune pellets. You can buy the Zupreem Natural, the Tropican, and the In-Tune pellets at any Petco, but the Harrisons and the TOPS, which are the best nutrition-wise, have to be ordered or purchased at a bird shop or another pet shop that carries them (Petco nor Petsmart do not sell Harrison's or TOPS)...What you want to do is to decide immediately on a Natural-flavored Pellet, buy a large bag of them, and provide the bag immediately to your breeder so they can wean your baby onto them...Or, if you breeder isn't local, just ask them if they can go out and buy a bag of the pellets at their local pet shop/bird shop to wean your baby onto, and then just pay them back when you pick-up your bird after weaning...Either way, that's the way you want to go...
Unfortunately a lot of bird breeders wean their babies onto a seed-mix as their staple-diet, which wouldn't be so bad if they would use a low-fat, high-quality seed-mix, but usually it's not, it's a cheaper seed-mix or a "junky" seed-mix that is very high in fat and low in protein, and this is the main cause of Fatty Liver Disease in pet/captive parrots and the main reason they die young...If you want your bird to eat a seed-mix as it's daily staple-diet, then you need to make sure that it's a seed-mix that is low in fat, high in protein, and that is "varied", meaning it has a wide variety of healthy ingredients...AS A RULE, if you feed your bird a seed-mix as their daily staple-diet, it should contain NO Sunflower Seeds of any kind, NO Nuts of any kind, specifically No Peanuts EVER, and NO Dried Corn of any kind, including Dried Corn Kernels and Dried Cracked Corn. These are the 3 main sources of carbs/fat in seed-mixes that MUST be avoided for a pet/captive bird who is not flying 10 miles a day like a wild parrot does to forage for their food each day...Sunflower Seeds, and I mean ANY TYPE of Sunflower Seeds, should only be given to your bird as an occasional treat (Sunflower Seeds make great "Training Treats"), and the same goes for all types of Nuts, they are occasional treats ONLY...And you should not EVER give your bird Peanuts, as they not only contain the most fat when it comes to Nuts, but they also carry specific types of mold/Fungi that is toxic to birds. So avoid Peanuts completely, and instead give your bird the occasional Almond, Brazil Nut, Filbert, etc. as a treat (Almonds are the best, very healthy)...And Dried Corn is pointless to feed your bird at all, it's nothing but fat and that's it, it's a starch that contains little to no nutritional value at all...Seed-mixes use Dried Corn Kernels and Cracked Corn as a "filler" to literally just take up space in the seed-mix. If you want to give your bird a little fresh Corn-on-the-Cob when you have it then that's fine once in a while...But no Dried Corn at all, certainly not in their daily food.
There are several commercial parrot seed-mixes that are actually pretty healthy and contain a great nutritional profile for a pet/captive bird. Again, they contain no Sunflower Seeds, Peanuts, or Dried Corn, but rather contain a varied mix of healthier, lower-fat seeds, grains, legumes/beans, herbs, veggies, greens, fruit, etc. A few of the most-common examples are Higgins Vita-Seed California Blend, Tropimix (which actually also contains Tropican pellets in it as well), Higgins Safflower Gold, Zupreem Smart Selects (not their Sensible Seed, it contains all kinds of fatty ingredients), and Higgins Vita-Prima No Sunflower Seed Mix...
As far as the Harness goes, this is absolutely another thing that you want your breeder to start getting your baby Tiel accustomed to right away, like right NOW!!! It is so much easier to get a bird harness-trained if they are started on it before they are fully-weaned, so when breeders are willing to start exposing their babies to a harness, and start getting them used to wearing them BEFORE they are fully-weaned and go home with their new owners, the birds are so much more comfortable wearing the harness and not even bothering with it. If you wait until your bird comes home to start harness-training him/her then it's not a problem, you can still obviously harness-train your Tiel, but expect it to take many months to get them to simply accept putting it on. Once that happens then it takes more time to get them accustomed to wearing it and feeling it around their body, and to get them to not constantly chew on it the entire time they are wearing it, regardless of where they are or what is going on...
****I would highly suggest that if possible you provide your breeder with a harness so they can start getting your baby at least accustomed to having it around them so that they won't be at all scared to it. Even just having it hanging on the outside of their cage all the time, and then allowing them to play with it a little each day, even if they aren't willing to start actually putting it on them and training them to wear it, this will do wonders and make the training process so much easier for you, because he/she won't be scared of the harness at all by the time you bring them home. Most of us use the Aviator Harnesses, they are by-far the safest harness made for your bird, the easiest to get them used to wearing, and the most functional...I believe a Cockatiel would wear a size "Extra-Small", but double check that...I have two Aviator Harnesses, one is a size Extra-Small that my Cockatiel, Green Cheek, and Quaker Parrot wear, and I believe the one my Senegal Parrot wears is a size Small, though I'm not sure about that, it might also be an Extra-Small too...But I know that the one my Cockatiel wears is a size Extra-Small...
****Just an FYI, if you want to take your Cockatiel out of the house at all, regardless of whether his/her wings are clipped or not, you must have them either in a carrier/cage or wearing a Harness/Leash at ALL TIMES...Even a bird with it's wings clipped can fly for miles in the wind. So do not EVER take your bird outside of your house without it being inside of a some kind of carrier/cage/Pak-O-Bird, or wearing an Aviator-Harness with the leash attached to you around your wrist, otherwise you will be very, very sorry.
