Andy,
You have a lot of questions! I am not a molder but a mold designer so I can answer a few of your questions. As far as sizing the mold to the press, it is a combination of factors. If your mold is too small, then you run the risk of crushing the parting line due to clamp pressure or peening the mold into the platens. Your platen size just limits the size of the mold. It is important to size your part shot size to the press. That way you dont leave material residing in the barrel too long.