Yes pleco's can scratch acrilic, but not all of them will. I would get something that stays smaller because as mentioned before a big Hyposthomus doesn't want to climb the wall to eat and wont do jack for you. Stay away from Panaque as they will definatly eat your tank! Ask to see if someone has kept the pleco you are looking for because if they eat driftwood their equiped to eat your tank.
Suggestions
L-116
L-231
Royal FORLOWELLA
Ptersturiosoma microps
Bushynose pleco's
Chaetestoma sp (rubberlip, bulldog, pittbull pleco)
If you have other small gentle fish with them you can also do a shiz load of
Oto's
Chinese Algea eaters are great on algea, but heed my warning. FLAT SLOW FISH are going to get their slime coat eaten. They do a lot of moving around and can stress some fish out. CAE's are actually a member of the loaches, and like high protien mixed in so they don't get the problems pleco's might if they are fed high protien.