Indian mudskippers are the only kind of mudskippers available that do very well together. They are considered a dwarf. African mudskippers need a lot of space, and then it might be possible to have two or three of them in the same tank, but they are much bigger and more aggressive than the indian mudskippers. You definitely would not want to mix the two. I think in the size of tank you're planning, one african or 4-6 indian mudskippers would be your best choice. Someone else here might be able to give you more info from experience, but that's my understanding from the info I've gathered.
You may want to check out the book Brackish-Water Fishes: An Aquarist's Guide to Identification, Care & Husbandry by Neale Monks. I just bought it and it has a lot of info on mudskippers.