I've used it before on plecos but only a half dose, and you won't get rid of the tank snails like that. Just disinfect the fish.
Best bet is to take the pleco out and give him a dip to remove any snail eggs he might be carrying. Then put him in a clean tank and keep him there until you are certain he is not a carrier. I don't know if snail eggs can go through the pleco digestive system alive, but I wouldn't doubt it.
Then disinfect that snail tank with a blowtorch and gasoline
( kidding...kidding... I'm just kidding!)
I like to use water and washing soda (sodium carbonate) which will disinfect the tank. Don't just wash it but let it soak for a while. If you leave it longer it kills more germs.
I think it is gentler on the silicone rubber and acrylic Plastics than common sodium hypochlorite bleach, but most people use bleach because few people buy Washing Soda any more.