Is it okay if I add a pleco or will these large water changes be harmful?
Water changes over 60% are generally not that great. I would upgrade the tank to a larger one first at any rate since the goldfish will soon outgrow the tank you have. They would get along fine and to work around the large water changes just do a few smaller ones more often. Like 50% twice a week.
I had a pleco and a goldfish together in a tank at 25 Celsius for a few years and they were both fine, catfish chased the goldfish around a little but otherwise they were fine. Unfortunately my goldfish died last week but I think it was due to old age and some rotten food in the tank.
60-80'F is fine for both species. but you need atleast a 40 gallon breeder before adding the bn pleco.