Good news: Pleco is doing great, and betta is getting better!!
As far as salinity goes, I was probably close to .3%. When I first set up the tank about 4 motnhs ago, I put in about 1 tablespoon because I've read that a little aquarium salt is good for fish as a general preventative. Whenever I do a water change, I add the proportionate amount of salt to the tank. Since I started noticing the ich, I added one tablespoon of salt once a day for three days, and then stopped because I was worried I was putting in too much (and also worrying about the pleco since you mentioned they didn't like salt very much).
I bought an antibacterial remedy specifically made for bettas with fin rot and other bacterial infections (BettaFix Remedy by Splendid Betta) which is "all natural" and has tea tree ingredients. I started using it on Saturday and there was almost immediate improvement. I think his fins are even starting to grow back!! He's more active and aggressively eating again (he never stopped eating, but he wasn't as energetic about it). Thanks so much for the help!