Mollies, and indeed most livebearers, mix fine with mudskippers of suitable size. You have to take care not to use any fish in the water that are the same size or larger than the mudskippers that you are keeping though. Mudskippers avoid going into the water when they see big fish because they are frightened of being attacked. Mudskippers can't swim well, so feel vulnerable. So use a smaller species of livebearer than the mudskippers; e.g., if you were keeping a mudskippers that gets to 6-8 cm in length, then guppies or Poecilia picta would be ideal. Mollies would combine better with the larger species of mudskipper.
On the flip side, mudskippers can and will eat very small fish including fry. The tails of fancy guppies may be nipped too.
You only real problem is this: 20 gallons isn't much space. Given that most of this needs to be land, only about 5 gallons, if that, would be left for the fish. You can't possibly keep mollies in 5 gallons of water.
Floating driftwood isn't acceptable for mudskippers. They need large areas of land, partly for feeding and partly for exercise. They are territorial animals as well, so a complex habitat helps them avoid one another.
The best thing is to create a mudskipper tank, and then see if there's enough water in the tank for fish, and if there is, add a few fish. Trying to convert a regular aquarium into one that accommodates mudskippers just won't work. If nothing else, the mudskippers will get bored and escape.
Cheers, Neale
Below is picture of a mudskipper vivarium taken from my
FAQ.