i got a 20 gallon long for my sump as well, i have my heater, my skimmer, and an overflow tube that goes to a separate 15 gallon fuge that has blacked out glass (used metallic paint to help reflect light) so that I can have the lights on during the night! More oxygen production! then the return pump is in the other side of the fuge separated by plastic plates so that the debris cannot get to it. there is not enought room in my sump for any kind of decent sized fuge, so that is the reason i got the 15 gallon tank as well.. u might want to go with a 25 or higher capacity tank for the fuge/sump.. nice thing about the separation of sump and fuge is the lights wont create tons of algae in the sump part..
i have about 2/3 of the sump full, if that tells ya how deeep the skimmer is.. i have a seaclone 100, and its finally working...somewhat..