I would seriously advise you avoid the "common" plec unless you are able to upgrade your tank in the future to cater for its massive size. Also even with good filtration plecs poop alot. An alternative would be a Bristlenose Plec. These in themselves reach a good 8" long under good conditions. To get a common plec with no intention of upgrading to a bigger tank as it grows is totally unresponsible. There are some fantastic plecs that stay under 8" that can be found pinned on
this thread here. It'll mean getting a far more appropriately sized plec for your tank.
Remembering that a large angel might eat small fish heres what i'd say about your shoaling fish. Personally guppys and mollies wouldnt be my choice because they drive me insane, however this tank is what
you want, i can only advise what i think will look good and be appropriate. If it were me i would have shoal of rummynose tetras, maybe 9/11...i'd have an odd number because it looks better
To name a few others tetras; Flame Tetra, Serpea Tetra
Another active shoaling fish for a centre piece i think would look good is if you were to get a shoal of say 7 dwarf neon rainbows. Stunning fish
Also your angels would be fine with them.
Corydoras for the bottom keeping the substrate clean by rummaging is a good idea, especially with sand. The upside down catfish ideally aught to be kept in bunch of 3, so think carefully if you can/want them. The angels as a centre piece are wonderfull, just becarefull not to have small fish with them if their big already. If there small they should be ok - but be warned
I had angels in with rummynose tetras and they were fine because i had the tetras first and were fully grown, then the angels grew up with them.
