It does sound like the pleco is the cause, I'd personally see if you can return it to the store.
With the goldfish, you'll want to keep the water quality really good and this will help their tails to heal
You can always use a liquid test kit at home, or visit a store for having your water quality checked, all you need is to know the ammonia, nitrite and nitrate levels. Ammonia and Nitrite you want to be 0ppm and Nitrate below 40ppm.
Also just be sure to keep up with maintenance, doing large weekly water changes of around 50% and be sure to keep up with cleaning the filter (just make sure you clean filter media in old tank water) they should be okay!
Keep an eye for any secondary infections, such as fin rot.