A pleco won't care about the substrate as long as it's got some driftwood to hang out on. (Are plecos ok with piranha?) Show us some pictures of your setup bumpy! If you've got slate on top of rocks, I think that might generate some anaerobic zones where the water doesn't flow and toxic gasses build up. You might want to go for a smaller diameter gravel until the piranha are big enough that their food is too large to fall between the pebbles.