I'm about to buy a 21' round and would like to know what to look out for? I want to know if there are things that would improve the pool like adding an extra return line or adding a center drain or anything similar? Most pools here come with a single return and I don't know if having 2 would be worth the extra cost? I would like to have some kind of fountain, so would adding an extra return be good for circulation? Is skimmer size important? Here, they vary from 6" to 14"? Is bigger better?