Baz, sorry we missed this one ... this is an interesting question.
Firstly, the general rule of thumb is that when you bring home a new bird to an already established "flock" there should a 30 day quarantine period of the new bird ... in this time a well birdie check with an Avian Certified Vet would be called for. You don't want any unknown diseases being transfer from the new bird to any members of the current established flock.
That being said, is that always how it happens - NO. Room limitation and time limitations some time make this impossible. If you can not quarantine then I would suggest these two things (1) know who you got the new bird from - have a good overall feeling about the new guy and (2) GET A WELL BIRDIE CHECK WITH AN Avian Certified Vet ... this is the only way to tell if your new guy is well enough to met your current flock.
Back to your original topic, can they get along - heck yeah they can (I know a gentleman who has a mac, 2 amazons, 1 grey, and 2 conures and they all get along splendidly). Always supervise interactions between the two new guys until you are comfortable that they aren't going to attack each other. If they accept each other then you have to do nothing ... it's only IF they don't accept each other that you have to step in and modify their behaviors a certain way ...
Let's hope they take to each other ... if they don't AD and I can help you understand what you will need to do to establish a good flock.