<<what happens when you power goes out and there is a siphon going from your main tank to you sump sitting below the tank>>
I'm setting up a new system, so I'll take a shot at this one. Whatever your overflow situation, it should be designed so that if the pump stops working the amount of overflow will be limited. An overflow box, for example, ensures that only water that can flow into the box will fill into the sump. Make sure the box is sealed well and that the overflow is near the top of the tank. If you drill a hole for overflow, do so very high in the tank. If you were to drill one on the bottom of the tank, for example, and not have an overflow box, it would drain the whole tank. (Giving you much worse problems than just a wet carpet.
) As for the return, the best thing to do if your set up allows is put it near the top of the tank. If you have a piece of pvc or other return hose pointed down, you can drill a hole in the top of the return hose. That way, if you get a reverse siphon going back into your sump, once the water level drains to the drilled hole, it will put air in the return and break the siphon.
As for the question about matching flow rate to return rate, you can choose whatever you like with your return pump. It's a closed system. The water will only return to the tank at the speed your return pump sends it back, so it will overflow at that same rate. Just decide how high you want the level of water in your sump, put the right amount of water in there, and then turn on your pump. It will circulate at the speed of your pump (or the speed you set if you use a ball joint). The only time this would be an issue is if you have a very slow overflow rate and a super pump. In other words, if you were draining using a 1/2 inch bulkhead and had a 1200 gph return pump, your overflow wouldn't keep up with your pump and would drain the sump or, if you handn't carefully set the water level in your sump, overflow your tank or both.
