Actually, gouramis are top-dwellers. They're labyrinth fish which means they need access to the surface to breathe. Sometimes, the males will build cool bubble nests on the surface as well. I'd recommend those because they definitely meet your criteria for color, but I'd recommend getting either just one or 4+. The first tropical fish I ever got were dwarf gouramis, and not knowing what I was doing, I put 2 into a 10 gal. tank. One harassed the other to death in less than a day. So getting just 2 is probably not a good idea; they'll concentrate too much on one another and can be aggressive. You may also want a combination of males and females (with equal # or more of the females); the females are less colorful but also less aggressive. I think you could probably fit 4 dwarf gourmis in a 55 with the stocking list you have now.
EDIT: Also, I have a question for you (original poster.) I'm thinking of adding 4 Boesemani rainbows to my current tank (55g) because 4 is all I really feel that I have room for considering their adult size (although I'll probably be upgrading in a couple years at the most; I think they grow relatively fast.) Do your 4 school? Do you know the ratio of males to females? I'm a little worried that 4 might not be enough for a school and they might get aggressive, because I've heard of Boesemanis being aggressive when kept solitary or in twos, so I'm just wondering how it's going for you with 4?