Male guppies are RIDICULOUSLY oversexed and will try to mate with anything--I've had them make a go at tetras, danios, even kribensis (although they each only try that ONCE!) Nothing will "happen", though. They're too distantly related to breed.
Scientifically, as regards the "fab four" livebearers--guppies, mollies, platies, swordtails--the following is true:
Swords and platies are in the same genus -- Xiphophorus. These interbreed very easily and most tail/color mutations in both species are the result of hybridisation. They will often interbreed in tanks of mixed species with no human intervention. Offspring are nearly always fertile, leading some people to ask if these two fish really seperate species or not.
Guppies and mollies are also in the same genus together--Poecilia. However, mollies used to be in their own genus--Mollienesia--and some people think they should be again split off. These only interbreed very rarely--even in 2006 the actual existance of this hybrid is very poorly documented. They will not interbreed naturally in a mixed community tank, and take a great deal of intervention to cross. The fertility of offspring is still unknown.
There can be no crossbreeding across the genus lines--Xiphophorus and Poecilia. Neither mollies nor guppies can breed with swordtails or platies, and the reverse is also true.
So 10/10 for guppies for trying.
