I love Rainbows! The newbie's Discus, lol, at least that's how I think of them
I have Austrailians, Red and Boseman's, love 'em all. My Aust. are HUGE (for Rainbows at least), like 5-6". The others are babies. They are hungry buggers, too, greedy. If it's just Rainbows, going on the old 1" per gallon rule, only 6-7 if you intend to let them stay in the 25 forever. They do school well as a species, btw, so you could get 1's and 2's of each of them and they'll school together.