Its never wrong to have a bigger overflow than return pump. If a snail decides to take a trip down the overflow for instance, you can have a wet floor. The Mag 5 will probably put out 350-400gph at an average lift, so you're probably safe with the 500. Add another powerhead or two in the tank for more random current.
55g reef, 2x250W 10k MH
+ 2x96W PC
actninc, 10g sump, AquaC Urchin skimmer, 65lbs LR
xenia, mushrooms, wels. open brain, fungia radiata (orange), green eyed zoos
flame angel, mated pair of false perc, 6-line, firefish
20gH electric yellow cichlid