the vinegar should take care of the poop issue ..vitamins are always good to have on hand stress, injury , shipping ,etc give some for a couple days..
Grit is to grind up fibers ..like BOSS(black oil sunflower seeds) or scratch grains or any other veggie any treat you give most are six weeks older for those kind of things.. supply it in a little dish they will eat what they want or need it is heavier and tends to just go to the bottom of a feeder. I know there is a chart somewhere i think one of the feed companies on what treats they can have at what age, if i find it i will pm you a link.
clean it see put it back in see if it gets picked on if it does you could always put in a small box in the big brooder so it is not alone..