its official-- 20 gallon long

OK this weekend im going out and buying a 20 gallon long tank or a 15 gallon. Hopefully the 20 if its not to expensive, because i love the size of a 20 gallon long, i think you just get so much more room and a variety of fish than a regular 20 gallon...anyways, i deffinatly want cory cats.
Ive set up the other posts but if you can tell me anything you can about them if you have them.
Such as temp, feeding, set up, filtration?
Ive never had catfish before so im sure sure if they need anything specific.
Also how many would fit in a 15 gallon? 20 gallon long?
Im sure a good amount good fit in a 20 gallon long, i dont really want the very tiny ones that only grow 2 inches, i like them more in the 3+ range. So what kind should i look for? I went to the real petstore today (not petsmart/petco) and they had a few different kinds to choose from, so if youve had good experiences with yours, let me know what kind.
Also i like very active ones! i love sitting and watching the tank so i like stuff to be moving, and ive heard they dash around if you have enough of them.
also i want gravel, not sand, i have sand in my piranah tank and i hate not being able to vacuum and have to clean the tank out every 3 weeks because of all the poop i see just disinagrating on top =/! i like being able to use the vacuum to get rid of the nasty stuff. Ill just get small, light pebbles since i heard they may burrow. In my old goldfish tank i just loved graveling it out! i felt like i was really cleaning out the tank and saving it from getting so polluted =)
Any help on this guys would be great!!!
Thank yaaa!