I haven't come across a mudskipper species happily living in full fresh.
Certainly the commonly traded species are brackish to full marine rather than fresh.
Due to their typical living conditions (estuaries) in the wild they are usually well adapted to fluctuations in salinity.
But hey, it's not hard to play with salt a bit. Just get a good refractometer.
I feed the skippers frozen insect larvae, a frozen mix of fish, shrimp and veggies - and common dried fish foods. They are not fussy at all, when it comes to eating.