Depends on the type of pleco, and your setup. There are many kinds of plecos that will work for smaller tanks and can be mixed--the commons aren't on the list, due to their large adult size. Bristlenose, rubbernose (also called bulldog), and otocinclus are good choices that stay relatively small. What do you want the plecos for?
With sharks--they can be kept with fish that do not resemble them in body shape. Plecos wil work, but SAE, CAE, and other sharks won't mix. They are very aggressive with one another, and territorial with most other fish, so providing them with a good cave to claim is essential.