26c is way too warm for a goldfish.
The tank is a good size for a goldfish or 2, but remove the heater and set it up as a temperate tank if thats what you intend to keep.
Back to the pleco though. You need to identify what kind. If you can't find out from where you got it, try and get a good photo and post it in a new thread and see if anyone knows what it is. Some plecos are fine in that size tank, and will generally be good in a temperate set up. However, some plecos get big. A common pleco might get to 2' and need a 400 to 600 litre tank.