ive got a shoal of 6 in my 90, and i dont have any problems, except when one of the twins scuffle over dominence, but it really is just when they make themselves look bigger than they are, and it rarely ends in a "fight," and then again it is usually a few pecks, thats all. the rest co-exist fine.
is your tank long or tall? either way, the angles should fight alot even if one iis a bit bigger than the other. if one is full sze though, and the other is puny, then there could be problems. in a longer tank, there is more room for them to steak yout terretories.
if you introduced them at the same time, you shouldnt have any problems once they find their terretories, because they will know where each others is and then they'll just leave each other alone. they might also become best buds and become unsperable, but it all depends on the personality of the fish. if you introduced them at seperat times, you could be in for a fight, because the one that has been in there could have possibly already clamied his territory