Sounds like a great tank! Have you thought about adding some larger rainbows? I find they really made a difference to a community tank - so much character, active and beautifully coloured. If you don't want to go that route, maybe some Kribs, they are cute and should fit in well with the fish you have.
Also, the cardinals are pretty but they don't have much character. A similar sized shoal of green fire tetras (or some other active shoaling tetras) will bring the tank alive.