Gouramis are anbenatoids like Bettas are, and many of them are also bubble nest breeders. There are many species that come in all different sizes.
For your 40 gallon, I think I'd recommend a small school of sailfin mollies. They come in several color varieties, so you can easily ell individuals apart, are livebearers, easy care, with intricate behavior patterns - the males dance and display for the females.
I'd go with 3 females and 2 males.
They'll want warm, slightly brackish water.
Then, to fill out the tank, I'd get a glassfish and a couple of bumble bee gobies. The gobies will help keep your molly population under control.