that's the same kind my Oscar the Grouch was... he was nearly 2 feet long when he died, so you might wanna upgrade him from a 10 gallon. Oscar lived in a 29 gallon with a common goldfish. bad mix, btw, but i didn't know any better. lived nearly 11 years, too.
A sail-fin pleco? Oh boy. You might have bought more than you barganed for. Juvenile fish need 3-foot tanks and adults need a 4-foot tank to be healthy (source). They are just as bad as the common plecos. I highly recommend returning him and searching for a bristlenose pleco. They are the plecos that can stay in a 10 gallon tank`(source).
Most if not all species (even bristlenose get a decent size) of plecostomus grow too big to live in a 10 gallon long-term. Plus the waste they produce is comparable to that of a goldfish so if your tank does not have enough biological filtration to cope you are going to see issues with water quality.
Even smaller plecos are big waste producers for their size.
If you want a pleco I would recommend upgrading to a 15-20 gallon and purchasing a bristlenose. While they still grow quite big it is not going to become a tank-busting monster like common or sail-fin plecos do.
Also I believe plecos need a natural source of wood in their tank as fibre or something similar so you might need to add a small piece of malaysian driftwood in.