I would use 1/2 schedule 20 PVC
if you can get it. It's thinner than the regular PVC
(sched 40) and consequently has a slightly larger internal diameter. I would use a globe valve if you can find one...if not, go with a gate valve. The gate valve will be easier to adjust and will cause less turbulence in the return line.
The Tidepool 2 is rated for up to 600 gph
). For a submersible pump, I'd probably go with a Mag 7. If you want to drill the sump and run an external pump, I'd go with a Blueline 30HD. Check out this page for info on pump specifications: http://www.captiveraisedcorals.com/pumpspecs.htm