Join Date: Apr 2015
Location: Westchester County, NY USA
IMHO the best population for big tank is either a very few large fishes (e.g. a pair of oscars + pleco), or a group/school of medium size fish (e.g. discus), or one/few large/medium size fish as a centerpiece + one/few big schools of small fish. May be also just a few big schools of small fish.
IMHO pretty much everything what you've listed is a poor choice for a big tank. A lost opportunity, if you wish. Honestly, I'd just leave all these guppies, bettas and dwarf shrimp where they fit much better - smaller tanks. Basically, the main thing is that in a big tank, only something big alone or big in a group looks really good. Small lonely fishes are just barely noticeable. Betta that looks gorgeous and a centerpiece in a 5 or 10 gallon tank suddenly is just a tiny spot in a 75g.
Corydoras may be a good choice IMHO. A big school of them. 10+ Obviously, not pygmy corydoras. Or a school of some reasonably sized loaches - and yes, all of them will completely destroy all snails. Do you really need snails? Then no loaches - there are lots of other bottom feeders (e.g. corydoras species you already mentioned yourself, plecos etc.).
Use big tank for something you can't keep in a small tank! Sparkling gouramis, betta etc. - you want them, get small tanks for them. There are so many options that you simply can't do in a small tank: discuses, altums, Boesemani rainbows, clown loaches - list of possibilities is huge.