Regular sailfin mollies are, well, mollies with sailfins. You either see the fancy sort for sale (orange, brown, white, etc.) or the regular sort (green with silver and orange flecks). One male and a couple females works well, and the males are super active and always fun to watch. The females look nice, just don't have the big fin. Together, they take care of algae to some degree, too. In theory these are Poecilia latipinna but they're more than likely hybrids. Maximum size in the wild is around 15 cm, though 10-12 is more typical. They're chunky though, and at no risk of being eaten.
Giant sailfins are Poecilia velifera. Males can get to 15 cm, females 18 cm, rarely 20 cm. These are BIG fish: imagine a molly the size of your hand. Wild-caught males have gigantic sail fins; tank-bred males stunt very easily, so the fin is sometimes not so big. They aren't at all common in the trade, unfortunately, but you may be able to get through mail-order distributors such as Sachs Aquaculture who ship wild-caught fish across the US for labs, schools and the like.
Cheers, Neale