has my pleco gone onto the meat eating stage?

Well as they get larger they do need more protein in their diet, however they do not ever enter a meat eater stage. While they do need more protein, they still will need to be on a large amount of veggies. 13" is a little small, generally, for them to become predatory, but it does happen at smaller sizes.

Most every common pleco ive had, has started scavanging once they hit around 5 - 7"s, but pretty much theyll eat anything they can if its there.

ive given mine meat at 4-5"s. they are omnivorous, but even at an adult they still need their veggies, they need the roughage. in the wild theyd eat anything anyway and have a balanced diet, and to get the most out of any of your fish, you should try to reproduce it to the best of your ability. good to here youre feeding both.