Here's the general rule of thumb. If you get a bird, you have to have the time to care for it in the event that it doesn't get along with your other birds. So, lets say you start seeing jealousy etc--- you may have to give your budgie separate interaction-time/ out-of-cage time from the others. If you do not have the time to do that, then wait until you do .
There are a billion factors that could influence the outcome and far too many to say that your birds
will get along (it's impossible to even predict if 2 birds of the same species will get along--let alone 2 different ones...then you have to think about age, hormones, cage size, cage setup, nutrition, sex differences, the behavior of the other bird, their relationship with you, individual personality, the season...the list of influencing factors goes on...)
You should also consider the age of your other birds-- I don't know how old they are, but if they haven't all gone through puberty, that could complicate your life a bit when they do (in terms of their relationships and how well they all get along), because if one bird gets aggressive, that could change the way you have to structure your entire routine (not just with a new bird, but also with the birds you already have). I'm not saying it's GOING to happen, but puberty causes lots of behavior changes in birds, and those sometimes impact their relationships with people and other birds.
You will need to quarantine any new bird carefully away from the others in a separate room for at least 45 days and get them all checked out at the vet, as even healthy birds can spread deadly diseases to others without showing symptoms...so a new one could bring disease in, but your current birds could also make the new bird sick.
A budgie is also VERY small compared to other birds and so you will have to be very mindful---people have had birds lose toes etc for landing on another bird's cage at the wrong time etc. Anytime you add a new bird into the equation, it has the potential to throw off the current dynamic.
Somebody was just posting about this (again-- not saying it can't be done but there is no way to say for sure)...
http://www.parrotforums.com/cockatiels/86408-how-introduce-cockatiel-2-budgies.html
see link above for that post.