Hmmm, what do you mean six inches max? The food? Sorry I am not responding to your specific variety of pleco. Mine is a mystery pterygoplichthys I got from PetStupid. But my story might help you.
It was about three inches long when it first went into a 175 gallon outdoor tank with lots of hides and I did not see it come out of hiding during the day for three months. When I did, it had more than doubled in size. The tank already had a fair amount of algae, but I think what I was throwing in for the platies always got finished up by the pleco AT NIGHT in the dark.
I don't believe in buying commercial preparations for feeding fish as long as they get good variety and things that are not known to be toxic, especially if the tank is outside where insects lay eggs in the water. That said, when the pleco started showing itself, it would eagerly suck up any food that hit the bottom. No big chunks, mind you. Just pinches of canned fish, shredded cooked greens, even some re-hydrated kelp leaves. I've since moved it inside to a smaller container but with more "swimming" room. The big tank outside was very very heavily planted.
It has become braver recently and now in broad daylight attaches itself to floating slices of cucumber, watermelon balls, thawed frozen cherries, (try to stay away from acid fruits), cooked sweet potato cubes, beet cubes, etc.
I think the trick with plecos are how reclusive they are during the day, needing lots of hiding places until they feel safe. I think seeing them as juveniles sliding all over the glass and sucking on vacation cookies in the LFS is deceiving. They are just dumb babies that soon grow to be very clever survivalist critters.