1) you can use a overflow box
http://www.aquamarin.com.sg/productsDetail.asp?productid=649
or
http://www.reefdepot.com.sg/AddProduct.asp?ProdID=314 to overflow to let it "suction and siphoning" it from main tank to sump tank.
else, if you wanna use pump and piping, you need to place one small pump from the main tank -> sump tank (another pump to pump water up).
However this is more risky as if one pump is not working, the tank it's in will be overflowing the other...
4) pump compartment can be normally 3, first compartment is the mechical filter, 2nd compartment can be refug or skimmer, 3rd compartment is water pump to pump the water back to your main tank.
Else, you can use a external canister which most sumpless tank will use.
HTH