1) freshwater... a lot of people will say they're brackish, which they will survive in.. but not as long.. they do best in freshwater.. as for pH make sure its between 7.0 and 8.0 these guys are extremely sensitive to change in water quality so make sure your pH doesnt rise or fall rapidly and clean clear water is essential.. with a 5g tank you will need to be doin water changes prob twice a week and checking pH every other day
2)5g for a dwarf is the bare minimum, 10g would be much better.. even though these guys grow to only like an inch long, they swim around a lot and are very active..
3)they're very territorial and need a lot of dense plants and caves.. if you have two, they will not live in constant view of each other so hiding places is crucial
4)deff no other fish besides the dwarf in a 5g.. these guys are very aggressive and will attack any species of fish besides their own, even if the fish is much larger.. if you got a 10 or 20 gallon tank you might be able to throw some otos in there, but dont be surprised if you find them chewed up and dead
5)there are several different common names for dwarf puffers but they are all the same species.. these guys pretty much all look the same.. theres no different color variants but you might find some with slightly different patters or brighter or darker coloration