I've never had any issues between cories and guppies. Cories really will never bother anyone IME. I would avoid bettas and mollies. That's not to say their isn't bettas and mollies out there that would be fine with guppies, but I have had experience with mollies nipping guppies, and bettas you want to avoid mixing with flowing fins in general. Some killifish will tear into fish with flowing fins, so use caution if you are interested in any killies. Of course there are many larger fish that could harm the guppies, but assuming this is a smaller aquarium, that's what I can think of to avoid.