I had a mudskipper way back in the mid 1980's. I used marine salt to turn the water brackish, but I cannot remember to what degree. Hopefully other members with brackish or marine experience can help out here on the salinity issue.
Aside from that, I would leave the mudskipper in the 20g on its own. They must have brackish water. And I used chunks of wood (Malaysian Driftwood or ironwood works well) to create a "land" area at one end of the tank.
There is a lot of info on this site
I fed mine live tubifex worms once a week, and frozen "fish" foods other times. Frozen bloodworms (but not too often), shrimp, squid were all eagerly devoured. It became quite a "pet" too, it always swam over and out onto the wood where I fed it, and ate from my fingers. It got so excited one evening it jumped out of the tank when I opened it to feed, and I was chasing it around the room.