One small thing that might be important, the canister size for the skimmer may be more of the problem than the rate. Since your sump is small, the large canister may "consume" too much of the total volume. If you add enough water to fill the canister and the sump to needed levels, the sump will overflow (this assumes the canister is on the OUTSIDE. Placing the caniser INSIDE would avoid this complication since it will displace the same amount of water that it holds (keeping the water level up but not setting up an overflow situation when it is turned off. A major benefit of putting the skimmer INSIDE the sump is that any small leaks (I have these set up both ways and have never had a problem but I have had problems with other brands) or cup overflows are contained in the sump and not on the floor.