Common plecos can live in sub-tropical to tropical temps, many believe that the cooler tropical temps are in fact more suited to them. But there are not any coldwater pleco's as far as i am aware, just a lot of tropical and sub-tropical ones. I have a common pleco in my goldfish tank purely because the temps never fall below sub-tropical temperatures (and also because it used to bully my other plecos, so had to be separated either way).
Many fancy goldfish are actually sub-tropical (and would die if ever exposed to freezing pond temps), so there are some plecos you can technically keep with them, but very strong filtration is needed (i would advise more than one filter in the tank, and overfiltering the tank at that) since both plecos and goldfish are high waste producers. However some plecos have been know to latch onto goldfish to suck their slime coats off, so a lot of care and a wary/stern eye kept over them is needed to keep them successfully.