Honestly, I'd think about returning the pleco. 19, 22....what ever. You need more room for him. I have one in a 75 with an Oscar, and he's getting nice and huge...along with the Oscar, but that's a different story. LOL
In short, I had both in a 55 and got a bigger tank, now I'm much happier with the results so far, because the fish are much happier.
As for feeding, just a plant weight or two. I have several tanks of various herbivours, and I just attach some plant weights...and drop them in. Also, don't skimp on the algae wafers. I throw a few in every night, after turning out the lights.