Sump/refug math..

You want the overflow rated higher than the return pump so the pump does not pump faster than the overflow can keep up. Keep in mind though pumps are rated at maximum flow zero feet of head so once you take into account the vertical lift and all fittings the flow is probably 1/2 or less the max or rated flow depending on which pump you choose.
A 600 GPD over flow would be more than sufficient fro a 800 GPH pump in other words since the pump will actually be doing more like 400 GPH, if that even.

No on the return and skimmer pumps. An in sump skimmer just pulls water out of the sump and retrurns it to the same place basically. Its more like a closed loop in a way. Another way to feed a skimmer though is to split the overflow stream so some feeds the skimmer while the rest dumps in the sump thus eliminating one pump and the heat they generate.

 

Alright, that clarifies everything, thanks!!