They not only all can get pregnant but if they have been in with the male for a day or more they may already all be pregnant. The hard part with livebearers is not having them become pregnant, it is keeping the environment good for them. If you are having water problems, it would be best to deal with that first.
A pH of 8 will not hurt the typical platy. A change of pH every day because you are trying to change it with acidifiers will harm them. The best advice I have seen on pH is that its best to leave it alone if its anywhere between about 6 and 8. The fish can adapt to the pH in the tank but daily changes are very hard on the fish. With a pH of 8 plus, I would worry more about the tetras than the platies. In general platies do well with somewhat hard water that has a pH of at least 7, but it can be higher without causing them any distress.
The nitrogen cycle needs to be well established but a platy is not a fish that needs a mature tank. It only really needs a cycled tank. For newly cycled tanks that need a chance to mature, a few platies is my favorite way to move forward. The platies do OK in a new tank and they set the stage for more sensitive fish to thrive later.