I was thinking the 3mm might be too small myself. I know we're talking about largely different sizes of fish here though, my smaller 8" or so ATF vs. pacu. How big would you say the 3mm are in actuality, maybe BB bullet sized? Just trying to visualize the size of them.
Mine takes food from the surface too, that's why I'm looking specifically at their floating pellet line. Mine isn't a slow eater at this point, although the silver dollars in the tank may have helped him with that. He actually usually grabs 5-6 food sticks in the time it takes the silver dollars to finish one each. So he usually can keep up without any issue (I normally put the entire amount I want to feed in at the same time, and that allows for him to grab a bunch before they can eat them all).
Just trying to decide between 3mm and 4.5mm for now, I'm not sure if the 4.5mm pellets would be too big for him to get down since they're not really chewable like the food sticks are.