Rainbows are fine as long as they are robust. I had a pretty large one about 4" in with a FH I was growing out and he was never bothered. Nics are territorial so he may chase them if they come into his territory. if youre on about getting a female then they are generally less territorial unless breeding. the female I had was really peaceful.
hope that helps
I have my female Nic in with 5x Rainbows right now. They get along great and the Nic rarely bothers them.