Never put your male betta in with a guppy. Especially males. He will think of them as a rival and will kill them for sure. I've kept male bettas with dalmation mollies successfully. You can tell the gender of a molly by the female will generally be larger than the male and the male will have a gonopodium.
You can read a little about sailfin mollies and determining the sexes here:
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I am not sure if all males would be aggressive towards each other, but if you get females from a LFS that keep males and females together, chances are you will end up with a pregnant female already.
I would just go ahead and get some mollies that don't have very fancy tailfins. Like a regular dalmation molly or a black or white molly. I would be scared to put anything too bright in the tank because of the betta. I would get males and females, if they do have babies they will probably have them while you aren't there and by the time you find one or two, the rest will be eaten by the other fish in the tank. especially the male betta. bettas love to eat fry.
I have a 30 gallon with a rainbow shark, a few platies and a few guppies and cories. My guppy gave birth to some fry and there were only 2 survivors I found in the tank.
If things don't work out with your betta, you can always put him back in his old tank. Make sure the current isn't too strong. If you can adjust the filter current I'd adjust it to the lowest setting. If not, you could try putting a pantyhose around the filter intake to slow the current for the betta's sake.
Just monitor their behavior for a few days and make sure everything seems okay. This does NOT mean your betta will never harm the other fish in the tank, but they should be okay if he doesn't see them as a rival or threat in "his" tank.
Make sure your tank is cycled before you put any fish in. Well you can do a fishless cycle with ammonia, or you can pick 2 or 3 hardy fish like guppies or mollies or platties are pretty hardy. I'd pick maybe 2 or 3 mollies to cycle the tank with if you want to cycle with fish, and then after 3 or 4 weeks your tank should be cycled. You can bring your tank down to your LFS and they should be able to test your water for you for either free or fairly cheap.
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Then after your tank has cycled, I would introduce your betta to the tank. Then maybe add another fish or two every week until you are fully stocked.
Or you can just buy like 2 female mollies and one male molly and then have the betta in there and see if you have an surviving fry then you can watch them grow up in your tank.