On the contrary, in my opinion, rainbows are definitely an option. For one, there are many species of various sizes. Your 75g is plenty big enough (that is 120cm tank), even for the larger ones, but maybe not with all your current fish. In my experience, rainbows are fine as singles or pairs in mixed rainbow groups. If you are worried about them being too big for the tank, considering your current fish, you can look into the smaller species. see the article: https://www.aquariumcoop.com/blogs/aquarium/top-10-rainbowfish. If you like the classic rainbow behavior and shape, the dwarf neon rainbows are great fish that only get ~3 inches long. Mine are very active and the males will "dance fight", but never physically hurt each other or bully each other to the point of detrimental stress.
One consideration though, is that the dwarf neons and the larger rainbows are very fast, strong fish, and go crazy for food, so you would want to make sure that your other fish are getting enough to eat.