it is a bad idea to get a pleco as a tank 'janitor'. it is a great idea to get one if you like them.
there are over a hundred species of the fish known as plecos. some are easy to find and nicely priced, others are rare and expensive. some are so common they are simply called 'common plecos'. a mix of three very-similar looking species (not interbred, just all three called the same thing), they will grow anywhere from 18-24 inches or larger.
plecos as a group eat a variety of things. the large majority only nibble on algae when it is abundant and they are hungry. otherwise, depending on the species, they eat fresh veggies, protein (shrimp, larger snails), or wood. most eat a combination of those things in varying amounts. expecting a large pleco to exist on algae alone, even if there is a lot of it, would be like you or me eating a complete daily vitamin pill and a head of lettuce every day, and nothing else. even algae wafers do not provide the amount of bulk they need to fill their huge digestive tracts, so they will always be hungry. most cases where people complain that their plecos are attacking other fish and eating the slime coat - those plecs are starving, expected to eat only algae and whatever food is leftover from other fish. i have found that when given the choice between eating a wafer or waiting until the next day for 'zucchini night', mine would rather wait and let the algae wafer rot.
i am not an expert, but i do have a lot of experience. i currently have 4 bristlenose plecos (ancistrus sp.), a Xingu royal (panaque nigrelineatus), 4 clown plecos (panaque maccus), a common pleco (gibbicep), and a rubberlip (chaetestoma thomasi). i also had a whiptail cat (rinelocaria) and gold nugget which were lost due to parasites after 6 months.
of course. . . . i don't have algae problems.