Its usually okay, but there is a slight risj of hybrids happening, so you shouldn't keep the fry fomr a tank of this sort.
Plus, the females may store sperm, making things more problematic.
You cna keep distantly related livebeares together though(guppies with sowrdtails platies with mollies) but not the more closely related types(platies with swordtails)
How many fish do you want, what colros do you like....any fin/tail shapes you want....do you wnat other fish?
Then I will be able ot help a little more