I think a school of five(that's the maximum) of the larger rainbowfish would probably be OK with your 55, ideally if it's a 48 inch long. Larger rainbows include australians, bosemans, turquoise, and others of that type. I've got a school of four (long story)bosemans in my long 55g, and they are happy. They do not bother any of the smaller fish, which are black skirt tetras and male guppies.
They do, however, occasionally uproot plants. It's not bad, but it does happen, esp with bunch plants.
If you are interested in smaller rainbows, threadfins are absolutely beautiful...I saw them for the first time today and am regretting that I don't have space for them.