First of all, three cheers for you! It sounds like someone has set up a goldfish tank that would be a good home for five or so adult goldfish!
Do try harder to do more partial water changes, gravel vacuuming some of the tank bottom. (Yeah, easy for me to say.) A month is not a long time to have an aquarium set up. If you can monitor the water by testing for ammonia (which should be zero PPM), nitrites (which should be zero) and nitrates (which shouldn't be more that 10 or 20 PPM) don't feed the fish as much for a bit. In addition to that, put in new, rinsed activated, carbon in your filter. (Leave the floss slightly gray so that there is still some beneficial bacteria there.) If you still have ammonia in the tank and you can afford an expensive ammonia absorber like Polyfilter, try that.
Probably this isn't for you but sometimes aquarists think that once a Nitrogen cycle is established, they are set. But it something we (me too) need to work top keep healthy and effective always.
No effective nitrogen cycle, no effective fishes.
Your pleco is a huge source of fish waste. Like the temperate water goldfish, they eat a lot of vegetable material and pass a lot of waste material. And he may double his size!
The scale loss may be caused by him (and dirty water encourages some aggressive behavior) but if there is some of that very toxic ammonia in the water, I would put my money on that and bacterial blooms.
Plecos need warmer water than goldfish really need. And keeping the tank at a pleco temperature causes the metabolism of all those fishes to accelerate and that means more fish waste in the water.
Could the pleco be given a vacation elsewhere so that you could see if that made a difference?
A "mystery disease" hit a rainbowfish tank I had. I had no clue what the disease was but knew that I had neglected recent partial water changes. I was lucky enough to have a food-quality 55-gallon drum (one of those blue ones) seated on some styrofoam (old fish boxes never die....) and kept at room temperature with a powerful submersible heater. The water was treated and aged.
I was fortunate to be able to give that tank daily 30% - 40% - 40% - 50% and 50% water changes. The daily death stopped. The obviously sick looked a lot better. Within a week they were leaving eggs (always a good sign). After the regularly schedule partial water change a week later, one could find fry in the corners (another good sign).
If I had waited to get an ID on that malady, the rainbow fish probably would have been all lost. If I had treated for an unknown disease, those fish probably would have died anyway.