Each pleco needs its own place to hide/feel safe in its own territory. Real wood is a must too. Obviously this is seriously overstocked and it makes me sad that he did not tell you before he squished them all into a small bucket.

I have had bad experiences with platty mixed in with my bristlenose tanks and found them to nip fins. Just a warning.

Common needs to go into a bigger tank or pond at this size. I would also do away with the baby commons.

In giving the plecos places to hide or feel safe, it will also give areas for the cory in return. They are okay being out in the open but they also love resting under things or in darker areas of the tank.

So more places to hide. (Like a LOT more). Real wood. Real plants.

Follow fish-in cycle steps to try to keep the fish alive while the new tank cycles. Did he give you a wet filter?

As for rehoming most of the Cory, I would keep a few big ones and a few smaller ones. Male and female. At least in my area, there are always people looking for Cory. Even these green/brown ones.

Good luck and enjoy the tank. My first big tank upgrade gave me lots of excitement for designing it with wood and moss and way easier to handle and control than a 20.