How many blues are you thinking? What sexes?
As for 'rainbows' - do you mean dwarfs (Colisa lalia)? The colorful ones are all male. The females are duller.
Dwarfs are considerably more peaceful than blue (three-spot) gouramies so don't often mix well. However, in a 90 gallon, you could fit a bunch of (4 say) female blues and a few dwarfs (say 2 males, 4 females) quite comfortably.
