I am having an 18x34 Doughboy installed. 54" sidewall, no deep end. My pump is a Waterway Econo-Flo VSA-165:
When I bought the pool, I did not know much about skimmers/returns. It comes with only one skimmer and return. The lead guy installing it mentioned it would be better for a 2nd return and skimmer to aid in circulation. He said while one would get by, my large pool would benefit greatly from a second, especially because there are a lot of trees around.
I called up the guy that sold it to me and his thoughts were that it would be beneficial, but not overly necessary because they up-sized my pump to one for an in-ground pool, and it should be more than enough to circulate the water well. He said he'd give the installers a 2nd skimmer in the morning so that they would have it in case I decided to add it. The cost is $130 for it, plus whatever they charge to install it.