Sorry to be harsh but...
If it is a common plec, I personally would not touch them with a barge pole. They grow huge and are poo machines. If you wanted a pleco I would have gone with afew bristlenoses, just as pretty and they breed very easily once settled.
Sorry to be harsh but I would take it back to where you got it and get something else.
You will never be able to get rid of it when it really gets big (although the pet shop may take pity on you and take it off your hands for nothing).
Also I would keep an eye out for a much bigger tank (200 litres+)for the labs, the tank size might have something to do with the aggression towards the Pleco.
Good luck