Finches

I have 2 sister Zebras that has to be separated because one would pick on the other.

I have found no rule of thumb with putting birds together. Sometimes they like each other and sometimes they dont. I think it has a lot to do with individual birds personalities and how they are raised. I have seen breeders with 40 birds in one cage that arent supposed to get along. Maturity and hormones will also factor in. Young birds will get along but as they become adults things can change. Also as someone else pointed out the mating or nesting behaviours can change things.

So if your birds get along them I would'nt worry about it so much.