Any livebearers (platies, mollies, swordtails) will all be fine with guppies.
Anything which won't nip at their long tails. I have 2 male guppies and also blue tetra's, cherry barbs and danio's along with a bunch of live bearers. Your local fish store should be able to guide you but if you want easy to maintain fish which all get along fine then livebearers are probably what you're after. Just make sure to keep the male:female ratio at 1:2 or 1:3.
Some fish are difficult to say - for instance I've seen guppies be just fine in tanks with Angels and loaches, but I've also seen them not live quite so nicely!