Sorry about your cory that sucks. Plecos dont eat other fish though as omnivores they will eat dead fish. Most likely the corey died of something else and the plecos did what they do, eat dead fish. Taking them back is probably a good idea anyway seeing as they will get too big for your tank anyway and are fish that produce alot of ammonia.
If one is worried about algae there are other more effective solutions. Contrary to popular belief plecos dont eat a ton of algae. To reduce algae decrease the time the light is on and also put the light on a timer so it goes on and off at the same time everyday. Algae needs 8 hours of continuous light to grow, any light even from a room light outside the tank. Plants dont need 8 hours straight to photosynthesize if one doesnt have plants fish dont need more than 8 hours of continuous light they just appreciate a schedule. Decrease feeding. Any food that goes uneaten produces nutrients that feed algae, any food that is eaten contributes to more waste produced by fish which contributes to more ammonia which contributes to more growth for algae. Feed every other day instead of every day. Lastly get algae eating animals. Nerite Snails will eat lots of algae and lots of different kinds of algae and wont reproduce in freshwater. Algae eating shrimp, Amano shrimp are renowned for eating lots of algae, ottocinclus dwarf sucker fish, they belong in groups of no less than 3 and will eat soft algae and diatoms they only get about an inch long, other snails considered pests by many but very useful in algae eating, ramshorn snails and pond or bladder snails.
Lastly again to manage algae do weekly partial water changes to remove or dilute the excess nutrients in water contributing to algae. Good luck i hope the store takes those plecos back for you and they find a bigger home.