You should never get a fish to clean the tank and eat the algae, a Pleco will not thrive on this diet and most won't even survive and alot won't even try to eat the algae. All Plecos should be given their own foods. Algae wafers, sinking pellets and for the Plecos with a tendency towards meaty foods Bloodworms and Brine Shrimp.
Not all Plecos get big, there are over 200 varieties of Plecos or Loricaiidaes ranging from the smallest the Otto to some of the largest like the Gibbiceps and Common Plecos. Most Plecos will not touch live plants either, all mine are in well planted tanks and they have never taking a liking to any plants.
Good Luck