I had a tank melt-down a while back and lost all my fish except for one longfin blue zebra danio. So I cleaned the tank out really well, put the danio back in and let it sit for a month... then I fell in love with a beautiful orange DSHM Betta and figured I would give it a shot.
So far they play in the filter current together and the betta and him play tag. No nipping/fighting/excessive flaring has happened yet and it's been about two weeks? I plan on either getting more danios, or giving my danio to a friend who has the same type in a much larger tank and getting some rasboras once I get a paycheck that isn't dedicated to unexpected bills.
I think all fish have a personality like people. So long as they get along, who cares if it's two species that "aren't compatible".