It's great you have a QT! You can QT say, 10-15 black neons in a cycled 10g QT tank at the same time, then after they're done with QT, maybe 5-10? more peppered corys. With the corys, if you need variety you can get long finned or albino peppereds (be careful there are few albino species, peppers are more unusual) but they know they are the same species
& will be more fun with more. Really, they are! & maybe they'll breed! Always fun to watch, even if you don't get survivors...but you might...especially with plants...(nagging just a bit, lol). If you really "need" another species of cory get at least 10 of each for maximun cory fun. The pleco will need a BIG hiding place or 2 under wood &/or rock structures. It has a strong sense of territory, corys just don't understand that concept & just bumble around in their cute cory way. I think I'd go with only 1 cory species...
For other fish with the pleco & angels, I love rasboras! (ok, not SA but, well, maybe you don't care about that, I often don't). I have kept & always like harlequin or espei, (similar), or red tailed (boraras petensis). All are peaceful, pretty & a nice contrast to your other fish. Tetras I would
avoid include serpae & "skirts", too nippy to angels long fins. Also tiger barbs. A school/shoal of 10-15 rasboras of the same species will look good & behave in a "normal" schooly way.
I haven't kept convicts (too aggressive for their size & breed way, WAY too well) or parrots, or eels of any size (I'm a loach & catfish lover first), so I'll let others answer that part, I have no idea...maybe the parrot with eel? I dunno...like I said...
As for your "common" pleco, it depends on its species. Some max out at 10 inches or so, other get much bigger...like 18-24 inches. Can you post a pic? I'm not the best at pleco ID but we'll help you or direct you to planetcatfish.