Do you have another tank that you can move these two fish to in order to treat them without subjecting the entire tank? Community tanks can be very tricky, because some fish, like your catfish, are more sensitive than others to various medications and salt treatments. Aside from water changes, I'm not sure what to recommend unless you are able to move them. I'm familiar with the needs of Platy and Mollies, but don't want to advise anything that could bring harm to the rest of your tank. Thankfully I'm not the only person on the forum! Hopefully one of our more experienced members will be by to advise. . .
I do water changes every week in my tanks, and in the tanks I run with gravel and livebearers, there is a noticeably
higher amount of waste that gets trapped beneath the gravel. I suggest that you try to get a water change and gravel vac in more frequently, if possible, and get the API Master Freshwater Test kit the next time you go to the pet store so that you can test the toxins in the water, and have a better idea of how clean the system is - then you'll better know if your maintenance routine needs tweaking.
Since these fish are so new to your system, it's difficult to say if they were ill when they got there, and stress from the move brought any of this about. Keeping a quarantine tank ready to screen new arrivals before putting them into the main tank, and to use in cases of illness is something worth looking into, if you don't already have such a system in place.