I'm pretty interested in the more 'rainbowfish' looking rainbowfish. Things like Praecox rainbows, and bosemanis.There's a couple varients of Boesemani that stay relatively small, about 3-4 inches, that you may consider. You can usually tell by the very dark vibrant colors when they are in breeding mood. Though typically the bigger and less-vibrant varient seems to be more commercially available.
Otherwise, you could consider featherfins or furcatas which are much smaller than praecox. If you were planning to do a community then you would have to do careful research if you consider the featherfins or furcata.