I've never done an F+D system so small, but I found with my system with 3/4" drains I needed about a 6" drop to get it to work properly... I generally have 12"+ on my drains, tho. For pump size, you generally want to turn over your tank volume 4x/hr, so 60GPH would be your aim. That being said, most aquarium water pumps do not run full flow with ANY head, so if you want to achieve 60GPH, you should probably aim for something that can push around 100GPH