Depends on the type of rainbowfish! There are quite a few.
I have bosemanis and some people say keep in shoals, some say you can have a harem (1m to 2f) etc etc. I had four (unknown sex) and when I added two more they got much less annoying to their tankmates and just squabble amongst themselves now.
I don't know if I'd say they're fin nippers per se but mine seem to be really nosey for want of a better word Always interested in whatever anyone else is eating or what they're doing,and when I first introduced my cories they spent a while trying to nip and butt them to make them swim off - cories were having none of it so they've given up on that game now
I have 3 western rainbows (1m 2f) and they dont seem to nip any fins. they are bullies and always take the lions share of the food but defenately not fin nippers. i feed mine blood worm every few days or so to keep their colours bright and vibrant!