I've never had any guppies go after other species of fish aggressively, but males are extremely annoying to everybody, including themselves, at times @.@ If anything, i'm sure they would only try mating with him a few..hundred times or so haha. Which will indeed drive him nuts. Female guppies on the other hand are lovely little darlings, and some actually do come in very fancy colors much like males. Just be careful your betta isn't super aggressive, or he may damage their delicate fins :/
Platies, on the other hand, i've had no trouble with at all. They're much more community friendly than male guppies (less annoying), and generally stick to themselves and swim about quite comically. You should get at least three to keep them happy, though this isn't a requirement, as they don't shoal exactly..They just like company. Keep in mind these guys are short but fat- they're actually quite sizeable for their small length, so you can overcrowd them fast.
I've never kept mollies, so I can't tell you about how they act. They are, however, pretty large for a 10 gallon tank (4-6 inches), and active, so it *may* not be a good idea to add them in with a slower moving betta as he may get overwhelmed.
Also, most importantly, be careful with ANY female live bearer you buy, be it guppy, platy, or molly, or something else. Any of these are likely to come pregnant, and continue getting pregnant for months even after there is no male. I hear bettas are very good fry eaters, though, so chances are not many of the babies will survive..But the sight may be sort of gruesome if you are faint of heart :( That being said, a female will be much happier with females than a male will be with males, so if you're okay with taking that risk I definitely advise it. Males can be a little rough with eachother, but will still do fine so long as there is no females in with them.