On the pleco eating, it is normal for such fish to eat "natural" food if it is available, so as long as you see it grazing algae, it is fine. When that is gone, as far as the pleco is concerned, it will begin to look for other food, and that is when prepared sinking disks or tablets that are algae, spirulina or kelp based are good. Veggies can be tried; not all fish will take to them immediately.
On the QT, I have become a won-over convert in recent months after twice nearly losing a tank of fish. It used to be that ich (white spot) was about the only disease we worried about on new fish, and it was/is relatively easy to deal with; but it was rare anyway. In some 15 years of buying new fish, I think I only ever encountered ich 4 or 5 times. But times have changed.
A professional microbiologist was telling me she is amazed at the increase in disease pathogens in fish today. Wild-caught fish tend to be free of these more than the tank-raised species, which should be no surprise. But of course most fish stores maintain circulating systems throughout their tanks, so any disease in one tank easily and quickly spreads to all tanks. And internal protozoan are for us impossible to detect until the symptoms begin to show.
I have also had half of the fish I buy die within a few days sometimes, but also after 3-4 weeks; only from one store, which I now do not frequent. But I also QT all new fish for 4-5 weeks, something I would have scoffed at a few years ago.
There are ways to set up a temp QT, but I personally find this cumbersome and impractical, so I maintain a 20g running permanently with a few plants and no fish except when new ones arrive. I am fortunate to have a dedicated fish room so this works best for me. This is an established tank, and with the fish in it I can monitor behaviours, feeding activity, and spot any disease. The QT period has to be sufficient to be useful, and a week or even two is not going to help much. My last group of Diamond Tetra looked fine, but 11 days after I had them in the QT they overnight developed ich, and it was massive. Two died within a day, the rest recovered and I kept them in this tank for another 4 weeks. By the end of that time, I was down to 4 of the original 8 fish. I shudder to think of the stress this would have caused the other 100+ fish in the main tank if I had not followed this practice. Plus the cost of treatments in such a vast space.