In general, your return line should be the same size as the outlet size on the pump. Your looking at it backwards. Your overflow has a flow rating, it's that rating (give or take) that your return pump should match (minus headloss).
So if you have a 1" drain hole (approx 600gph) then your return pump needs to return approx 600 gph
to even everything out. Your pump will need to be rated higher than 600 because it will suffer headloss (lose power as it needs to pump the water upward). Each pump has these rating as different heights. You can use a much bigger pump (say a mag12) and put a valve on the return side to dial back the flow or you can put a T with part of the flow going back to the sump to even it out (with a T also to fine tune it).
is fine. The more curves, elbows, couplings, etc will cause more headloss. I prefer flex pipe in and out.. just easier to deal with IMO
but to each his/her own