Welcome to Fishlore...Congrads on the new tank.
I wouldn't suggest getting a small pleco for a 10 gallon..I just think they get to big even being the smaller of the plecos and have a big bio load...
Cories do like to be in schools of at least 3 but right now your tank isn't cycled and it's not safe to add any more fish...
You have a couple of options..You can take all your fish back and do a fishless cycle which is super easy and we would all help you out
or you can cycle with the fish in the tank...Here is a link you can read that will help you understand what the cycle is
http://www.fishlore.com/NitrogenCycle.htm
If you choose to cycle with fish you'll need a
test kit..I would recommend getting the
API liquid master test kit..It's very accurate and overall cheaper...You'll need to do daily 50% Waterchanges to keep the
ammonia and
nitrite from harming your fish. If you can use Prime or Stress Coat plus as a water conditioner..They make the ammonia and nitrite safe for the fish for 24 hours..but still allow it to be used by the bacteria..
This can all be a little over whelming at first but good for you for asking questions and trying to do what's right..Just keep asking questions on things you dont' understand and you'll pick up on it pretty fast. Good luck.