no need to change to a different schedule. once a week is good until your snake pretty much full grown, then you can slow it down to once every 10 days or two weeks even.
at 20 inches long it really depends on how big around it is. If one fuzzy leaves a lump that's visible for two full days it's still a good size, if the lump is gone before that time, try two fuzzies or a smaller hopper. keep in mind the 1.5x rule, I've successful fed prey 2x as wide but it's probably not a good idea if you can avoid it.
As for the kind of corn snake you have: The spots down the back are often called saddles, if he's got black circles around the red spots it's probably a classic (normal) corn snake. Pet shops usually sell normals or a few of the more common morphs like amel, anery or snow. A lot of breeds will sell their high end morphs at reptile shows and sell the more common ones to pet shops.