What Newman says is true. They are likely pregnant already. However, I read somewhere in the past that if you want to breed a particular pair of livebearers that you need the desired pair to be together the first 24 hrs after the female delivers a brood. Apparently, the females can be receptive at this time to prioritizing the sperm of a new beau. So, if they are together now, you'll be taking advantage of this automatically.
As far as the amount of hifins. Well, this depends on how fancy your male is. Some of the really extreme hifin males are incapable of mating. My lyretail sword males have this issue. In which case, you need hifin females and regular males. This will likely result in about a 50/50 hifin/regular fry. However, if you got your male from a petstore and not a hifin specialty breeder, its likely his gonopodium is functional. I just bought one that I am pretty sure is capable of breeding. He is nowhere as fancy finned as I have seen from some breeders that specify that they sell you normal finned males for breeding purposes.