Are you sure there is anything wrong with your fish, It's easy to spend too much time looking for problems when none really exist. I have done it, as have many others. With Pleco's it's simple. Is his belly concave or convex. and how long have you had him. If his belly is convex and he has been in your care for a while he probably doesn't have internal parasites. Pleco's usually don't handle the stress of acclimation well and if there are any other health issues at hand they will manifest within a day or two of moving them home. If pleco's eat good and seem comfortable, they are usually fine in a decent clean environment. IME IF you get a pleco home and he eats well, and he lives for 30+ days he will live until you do something stupid and kill him. The first 30 days seems to be the kicker for me anyhow.
metronidazole soaked into his food is the best bet if he is still eating. If he is past the point of eating then Met in the water column may help, but isn't always very effective. Praziquantel is also a good med for many parasites but without a diagnosis it's hard to say which med is better. I would start with Met personally soaked into the food and go from there.
As far as the other fish dying, I would still suggest researching gill flukes. Flukes usually open the door for other problems rather than killing the fish themselves. The description of your angel that died sounds like Fungus or bacteria to me. In a crowded tank flukes will become more of a threat if they exist. Praziquantel is IMO the way to go for flukes, very effective, and little to no side effects that I can find.
BTW, Metronidazole has little or no side effects either.
Dave