Hi,
I have just set up my tank for mudskippers, just waiting to source some. It's not been too hard to do. The only thing is that they like to be able to burrow so the substrate needs to be fine sand, we have had to use rounded stones to keep it from sliding. Rounded stones are required as they have soft bellies and sharp objects can cause injuries. Also, if you plan on using drift wood, which is ideal for perches then you need to bury the ends.
Dwarf Indians are the smallest at approx 2/2.5" when fully grown. They are also the easiest to keep in groups as they are the least aggressive. You can get Africans which grow to about 6" but are the most aggressive and best kept singularly or the silver-barred which grow to about 8" and are ok in 2's but need a lot of land space due to their size.
We have a 60 litre tank with 25 litres of water and the rest land. We are keeping some mollies in the water with them as they like brackish water and the mollies are roughly the same size so won't be viewed as either dinner or a threat. Too big and the mudskippers won't get in the water and they need to send about 10% of their time in there. I have heard of people keeping fiddler crabs with them but this is not always a great idea as the fiddler crab is a natural food source and the mudskippers will eat them especially when the crabs shed.
For our tank we have an external filter suitable for tropical/marine fish. The heater is just a standard heater set to keep the water at about 27 degrees which is the mid range of their temperature. As mudskippers spend 90% of time out of the water they need high humidity so you need to make sure your tank has a good fitting lid, we have put tape over the holes where the wires for the heater and filter come out in order to keep as much humidity in as possible. Also Dwarf Indian mudskippers can climb glass so this is another attempt to keep them in the aquarium as I have heard of cases where they have gotten out of these gaps and it has been too late once they've been found!
We have some bacopa compacta and java fern in the water but this is more for the mollies than the mudskippers.
Hope this helps.