sump help

[Aquattro]

Rachel, a MAG 12 will be sufficient for your return or a skimmer. You could go smaller depending on skimmer design. I think Bruce's skimmer could get away with a MAG9.5.
The advantage of the MAG pump over the little giant, aside from cost, is that it is submersible. This comes in handy.
Whether you need 1 or 2 holes is a matter of preference...I use 1 for an overflow and have my return come up and over the back of the tank. I use a 1-1/2" pipe for my overflow, but it reduces to 1-1/4" at the bulkhead. This causes some back pressure in the stand pipe and helps quite the Durso style drain.