What types of turtles can be kept together in the same tank?

I've kept many different species together myself. Just trial and error really. I find the American pond turtles (sliders, painteds, cooters, etc) too aggressive for the small Asian pond turtles as well. Maps seem okay. Softshells, snappers, FRTS, mata matas and snakenecks are a no no for turtle communities.

i dont think frts, have EVER been illegal to own in the US.
I think you just cant import them.

IIRC FRT's are listed on CITIESII which means they are illegal to EXPORT from their native habitat, but not illegal to import from other areas or own. Like SimonL said, cohabbing is pretty much trial and error. I had a RES and Florida Peninsula Cooter together with no problems.