Just a suggestion:
I don't know if your 20g can fit under your 75, but if it can, place it there and go to your LFS and have them drill a hole at the bottom for a return. In that spot you would add a pump to return your water to your tank via PVC plumbing.
Get some eggcrates and build shelves in your 20g and add filter substances (Small live rock, filter media of sorts etc. .) get with your LFS and ask them for ideas. Have your overflow run to your 20g, you can do this about 1 million different ways. I would have it make some sort of spray over my filter materials.
Add your skimmer into this mess however it fits. Just take this basic layout to your LFS and have them help you come up with ideas on how to procede making this work for you. And if you don't have enough room under your tank, you can always set it up next to your tank and make it a soort of refugium.
As you can see the possibilities are endless. Don't be afraid of being imaginative, running lots of water through, over, out, and back in is never a bad thing and with a sump you have LOTS more option.