You should try to return the common pleco to the store. They get to 12" and produce a lot of waste. They are completely unsuitable for a 10g tank.
As for the 1g tank, did you cycle it? Of course, you need to make sure the temperature is roughly the same as the tank you're moving fish from. With a tank that size, you would probably need to do water chnages two or three times a week to get healthy fry.
Yes, you can put a male and female in a tank together. You need to remember that livebearer females can store sperm, so you will need to leave just the ones you want to breed in a tank for several months before you're sure you're getting the offspring of those two fish. You don't need to put them in their own tank if they're mating, only if you want to make sure two particular fish mate.