I think you have a bacterial problem going on.
Probably combined with ammonia problems. Ammonia is toxic at very low levels when the pH is above neutral.
Ammonia burns the tender tissue in the gills and the fins, so the ragged fin-rot look could be from ammonia. It also is very stressful to the fish and this can make them more susceptible to diseases.
Whether the tissue is burned by ammonia or injured by a bacteria, fungi can move in on the damaged tissue. The tips of the fins that look a bit white are typical of fungus moving in on fins that are compromised. (Second picture- looks like a Platy)
Mollies and Guppies are fine with salt, and high pH, and hard water. I would not use pH down with them. Salt can help them deal with stress. These fish can handle a lot more salt than live plants would be OK with, so dose a small amount and see if it helps. 1 teaspoon per 10 gallons is a fairly low dose, and many plants are OK with that. You could double it, but maybe take some cuttings first and save them in another tank.
At this point I would do a couple of really big water changes with the goal of getting the ammonia under control.
More things to help with ammonia:
Feed less. If they will eat a food with antibiotics that would be the best.
Add nitrifying bacteria. Read the label and look for Nitrospira
species of bacteria. Nitrospira
is not actually the one that handles ammonia, but companies that include it in their cycle products also have the correct ammonia removing microorganisms.
Add lots of live plants. Even if it is just trimmings from another tank. Floating near the light, these plants will take in plenty of ammonia.
Use a dechlorinator that specifies that it locks up ammonia. Make sure your test kit is compatible with the dechlor, so you know what the Ammonia test is telling you.
Never combine meds unless both are labeled for use with each other.
By treating them with oral antibiotics there are less antibiotics in the water, so the nitrifying bacteria can make a recovery and get going on the ammonia problem.
If you need to treat the fish by adding meds to the water then the nitrifying bacteria will not reproduce so fast, so ammonia removal will be up to you for a while.
Although we recently added the guppies.
Always use a quarantine tank!