Dwarf gouramis would be compatible in your tank. The main problem I have with them is finding good stock. They tend to be really overbred and you'll see a lot of low quality and unhealthy ones. If you can find good stock though they'd be a good choice.
As far as rainbows go, it depends on what species you get. The australians do get pretty big, and the boesemanis a bit smaller than that. There are some like the dwarf rainbow that only get a couple of inches long. You just have to hunt around for what you want.
There are a lot of tetra species that are pretty hardy and would be just fine in your setup. A lot of people like black skirts, and they look different than the standard tetra and don't get very big.
Glolights are pretty neat looking and easy to take care of. Rummynose are an attractive red, white and blue-ish/black color. Neons, which are pretty common, could do ok in your setup, but I tend to stay away from them due to having trouble finding healthy stock.
You could actually do an angelfish, or a pair, in your setup since they're not super active. I have one in my 220g aquarium and he just sits in the same corner of the tank all the time. Just make sure not to buy altums, which get too big, though they're not commonly found in aquarium stores anyway.
I dunno, there's just too many fish to think of. You could even do a group of female bettas or something.