DEFINATLY no fin nippers! These includes Danios and barbs...
Do not mix with Gouramis of any kind, as they will fight to the death... Also, NEVER have two male Bettas in the same tank unless they are seperated. Don't have males and female Bettas together as they also fight.
Small Tetras can be fin nippers, but I've never had trouble with them as they are very peacefull placid fish. Corys also make good tankmates.