I love this build. I found it awhile back and plan on doing something similar for a large brackish paludarium.
has a few mudskippers in right now, I want to get the whole group when I have my 30 open up. And eventually do a 55 or 75 with some Knight Gobies and Bumblebee Gobies in it as well.
Anyways, maybe buying a few more would show some more of their behavior. A 20 long would probably be good for 2 or 3 of them.
If you want to get Knight Gobies I'd upgrade the tank. Mudskippers don't get in the water very often, and if everyone has their own territories you shouldn't have to worry about them attacking each other.
They do seem to be good tank mates, almost all of the paludariums I've looked at successfully keep them together.