Since I built my stand too darn small I was forced to get inventive with my 20gal sump. I route the overflow water to the far side in an open area which holds the heater, some smaller LR pieces with macroalgae growing on them sitting directly on the glass bottom of the tank, then some CC just about an inch deep with broadleaf caulerpa growing in it. Then I have one of those perforated tank dividers keeping it all on its side of the sump. Sitting snugly against the divider is my AquaC Urchin skimmer... this is set to take water in from the middle of the tank and return it toward the far side wall. Within this same area is the return up-take. I have this plumbed with pvc ended by a pvc tee since I thought this would both keep the intake up off the very bottom as I noticed it was suctioning itself to the floor, and it [theoretically] allows for one side to get completely clogged by an errant snail while still having full uptake capability.
I wanted to have the skimmer after the caulerpa to allow maximum nutrients to flow through the planted area, don't know if I did right.
I hope to add more substrate, and have considered having a small container with mud, etc within the large planted area, which might help encourage critter growth and keep the mud from washing away.
All this is poorly lit with a dual 20w NO flourescent fixture 24/7. I hope to get a higer output lamp some day and switch to reverse cycle with an overlap time so there is no abrupt change in pH, [again a theory].
I have a two piece hinged glass cover with the front pice cut to allow the skimmer to extend out since it is taller than the 20gal tank. This seems to help control excessive evaporation, and I have heard that there are other benefits to having a covered sump when it is planted.
Just a few critters in the sump so far, a few snails and one blue leg crab, plus whatever unseen things hitchhiked in, probably a bristleworm or three...
Note that the return pump is mounted outside the sump and next to it within the stand. I have an Eheim and like it very well, but it seemed to heat up the water, especially during this hot summer. More room in the sump tank without it anyhow, so I'm happy.
Hope that helps you come up with your own personalized plan for your situation.
kjb