I wouldn't get a pleco for the sole purpose of cleaning algae. All of them stop eating algae eventually, and algae isn't the only thing they need to eat. bristlenose plecs need some driftwood to suck on and plenty of veggie content, mine gets lots of lettuce and cucumber to help him stay healthy. He's growing lazy on the algae, but that's nothing a good ole brush can't handle. cleaning algae off the sides of my 75 gallon tank takes like 2 minutes.
Aside from this, they're very, very messy fish. In place of eating algae, they poo none stop, dirtying up the water faster, thus making water changes needed more often. My pleco is the sole provider of every string of fish poo on the bottom of the tank, the smaller, lighter strands from other fish get sucked up by the filter. IMO this is a much bigger problem than a bunch of algae.
At work I need to clean algae off the walls in the pleco tanks every 4 or so days and bleach decorations at least every other weak or else they turn black, cause they crap a lot faster than they eat.
If you like plecos cause they're cute and fun, get a pleco. If you want a pleco cause you don't wanna take a couple minutes to wipe algae off the walls, don't.
And yes, otto's do need to be kept in groups. but 6 ottos is not as messy as 1 pleco.