Turtles and tropical fishies...
 

...Are they compatible? Can they go in the same tank together?

As a rule, no. Turtles eat fish. People also buy rosy red minnows and small comets to feed to their turtles.

I keep my painted turtle in a 75 with about a hundred feeder guppies. Whenever he gets peckish, the guppy population drops.

Before I figured this out, when he was just a tiny thing, he killed four cories and a half dozen guppies. I figured Banjo cats would dig themseves into the substrate and stay out of his way. He dug them up and ate them.

No, turtles do not mix with tropical fish.