I think bulldog plecos are typically supposed to be housed in groups, but I could be wrong.
You definitely have a 'common' pleco, which is the big kind. If at all possible, I'd look into rehoming the big fellow. Bristlenose plecos will get around 6" long, if that, or you could try a twig catfish. Twig catfish are about 7" long at full size, but they're very slender and mostly tail, so they don't take up much bio-load space. They'll eat algae and will happily devour veggies or nori when the algae runs out, plus they're awesome-looking and very gentle. I suggest you look them up.