Spotted suckermouth catfish compatibility?

I have a 20 gallon tank, with one spotted suckermouth catfish who keeps the tank nice and clean. I recently gave away my cherry barbs (since I found their nippiness stressful) and replaced them with guppies from a friend. I'm also planning on getting some red cherry shrimp. I heard that livebearers get along, so I'm wondering if I could also fit in some sailfin mollies, but I don't know if they'd require water that's too brackish for the catfish and shrimp. I've read different things about how much salt they need, and I don't know how salt-tolerant the other things are.