I’m by no means an expert and won’t comment on the cause, but I will say in my experience corys and plecos are the last ones to show signs of the problem but the first ones to struggle from medications. If you can separate them for treatment I would. One of my worst days is when I inadvertently killed a couple plecos while treating a nasty (finally figured out...I think) columnaris infection from a really popular Lfs. I still call it my “death tank” and always have regret when I look at it. Also, I totally freaked out and went crazy on treatment, which is probably the worst response. I’d use every available resource to try and really pin down what’s happening and treat appropriately.
in a pinch, you can relocate to a plastic bin with an air stone and a usb pump from Aquarium Co-op for the catfish and work on saving the danios.
good luck!