The skimmer seems a good deal. It comes from Hong Kong so you will not get a lot of support, but for the money it seems an option. However, at $74 for shipping you are at the $200. For $80 more bucks you can get this:
http://www.marinesolutionsinc.com/c...Skimmers/c22_48/p132/MSX200/product_info.html
Your overflow is gravity fed, you want it at least 2-3 times the size of your return pump GPH. I use a 1-1/2" overflow (2100gph) and return water at around 600-700 gph.
There is a difference between the amount of water that you can move for a specific diameter pvc when gravity fed vs the amount of water that can be moved for the same size pipe if it is pressure fed. Your return is pressure fed with the pump. At one point you need to consider volume vs velocity.
For example, you can move 500-600 gph in a 1/2" pvc pipe but it will exit the pipe at high velocity. The same 500-600 gph in a 1-1/2" pipe will move slowly. High velocity has advantages, you can use that velocity to produce movement inside the tank, but also produces more noise (more friction). In my opinion you are better of running 3/4" pipe (either flexible pvc or even braided plastic hose) from the return pump and then limit the output with a Eductor to get higher velocity.
And to answer you first question.... yes, you control everything by throttling the return, either by adding a valve on the return or by buying a lower GPH pump.
Keep in mind a couple of things in your design:
1. A higher GPH return pump gives you more options for future expansions, like adding a media reactor. You can split the return and run half of it through a media reactor and the other half back into the tank.
2. Use True Unions and True Union Valves. There are more expensive but totally worth it. You will need to clean your return pump at some point (recommended at least every 6 months) and being able to just close a valve and quickly disconnect a pump is great.
3. Which brings me to the 3rd thing to keep in mind. Siphon effect!! Your return, when the pump is off (electric failure) will siphon out you main tank. So in reality, whatever is the lowest pipe output in the tank that will be the level the water will reach in case of a power failure. So you either add a couple of anti-siphon holes in your return or take into account that extra volume of water to leave ample space in the sump.
Bittersweet.
