From what I've read and seen, after you put the new bird through a quarantine period, it's ok to house them together. It all depends on the birds' personalities and whether they enjoy living with each other. I've read other member's experiences with introductions haven't gone as smoothly as they thought they would. Maybe they can give you some pointers when you're ready to try. Mine have always lived together cuz I got them basically at the same time.
The only thing to keep in mind besides that is that if you put a male and a female together, there's always the possibility they might mate. That's something to think about if you're not planning to breed them.