What are the tank dimensions (length x width x height)?
What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
-------------------
Mudskippers come in 2 sizes.
King mudskippers grow to about a foot long and will bite your finger off if they get a chance. They have big teeth and are quite intimidating.
Normal mudskippers grow to about 4-5 inches and are much nicer and really cool. These little guys tame down incredibly quickly and learn to take food from your hand or fingers and don't normally bite the hand that feeds them, although sometimes they miss.
Mollies come in a range of species, colours and sizes. Sailfin mollies (Poecilia velifera) are the biggest and can reach 5-6 inches long and the males have huge dorsal (top) fins. P. sphenops being one of the smaller species and grows to about 3-4 inches long and have smaller fins. They all come in black, white, gold, green and a few other colour forms. A word of caution with common livebearers like mollies that come from Asia or any fish farm. They usually have intestinal worms and gill flukes so should be treated for these before being added to a display tank.
There are a couple of small blue-eye rainbowfish (Pseudomugil signifer and P. cyanodorsalis) that do well in backish water. P. signifer come from a range of waterways including fresh, so you need to get ones that have been caught in brackish water for your tank. P. cyanodorsalis naturally occur in estuaries and the ocean in sheltered bays and harbours. They are quite stunning fish when mature and kept in brackish or sea water.
Celebes rainbowfish would be alright in a brackish tank and actually need hard water with a bit of salt to survive. You could keep them with P. signifer and mollies.
As a general rule, marine/ brackish water pufferfish should be kept on their own or in breeding pairs. They eat anything and if they get stressed, they release poison into the water and kill everything in the tank. If you keep marine pufferfish, it's a good idea to have a carbon filter running in the tank and to replace the carbon regularly (every 2-4 weeks). If the pufferfish don't get stressed, they don't release the poison, but if they do get stressed, the carbon will hopefully remove most of the poison and give you time to do a huge water change and find out what is stressing the fish.
-------------------
Mudskippers need some land to hop about on. You can make floating platforms for them or have part of the tank with a glass panel and shelf, which can be covered in sand and the mudskippers can hop about on that.
Some people have 2 tanks for mudskippers. The top tank contains the fish and has a couple of drain pipes in it. One drain pipe is high and sits about an inch from the top of the tank. The other drain pipe sits about 6 inches above the bottom. The water from the top tank drains into the bottom tank and a couple of pumps push the water back up to the top tank. It's basically a trickle filter/ sump set up.
Several times a day one of the pumps turns off and the second pump keeps going. The water in the top tank slowly drops down to the height of the second drain pipe. The water remains here for several hours and the mudskippers can come out of the water and hop about on the land for a few hours. Then the second pump turns back on and the top tank fills back up.
Mudskippers generally do best in their own tank, and in fact most of the fish mentioned do best in single species tanks. You can mix the blue-eye rainbowfish together and P. signifer would probably be ok with mollies. P. cyanodorsalis are small fish and would get eaten by mudskippers and probably be stressed out by mollies.
The following link has information on all the known species of Australia and New Guinea rainbowfish and includes Pseudomugil signifer and P. cyanodorsalis, and might interest you.
Melanotaeniidae and Pseudomugilidae
