Yes Andy, the pandas are corys. There is no definate number that corys should be kept in. You can keep 2 but corys are much happier in larger groups therefore people suggest minimum of 4-6. When kept in a group you will notice the change in personalities. I have 8 Cory in my 33 gal tank but could go to more like 12-15 if I wanted. It's fine to mix the corys, they don't have to all be pandas.
Regarding the sharks, I'd take them back and tell the guy he doesn't have a clue what he is talking about! Keeping a fish in a tank that's to small will stunt it's growth, in effect he is right in saying it will only grow to the size of your tank but what he didn't tell you is that although the fishes body will stop growing, it's internal organs won't meaning the fish will grow deformed and have a much shortened life. Silver sharks kept in the right conditions should easily reach 15-20 years old.
NEVER trust any advice given to you by a fish store, there first priority is to get you to spend your money! Always research fish for yourself, ideally on this forum on the Internet and also never impulse buy fish.
I'm not having a go at you because you obviously weren't to know and we have all made a mistake in the past by trusting our lfs on one thing or another!
Andy
