Nitrites shouldn't be showing if your tank was properly cycled. Yes nitrites can harm and potentially kill stock. I've had quite the opposite experience with pleco. I've never had one that doesn't love vegetables. Also, the veggies are great for them but do not contain all the nutrients the pleco needs to have in it's diet like the wafers do. My tinfoils used to eat the damn logs I put in too so I'd just throw in enough to fill their mouths and then a couple extra. Or get a large one that the tetras can't eat.