If you are just feeding chick crumbles, they don't really need grit, but if you want to start giving them treats (way fun), such as grapes or veggies or - yum!- worms or crickets, you should give them grit. You can buy chick grit, which is just a very coarse sand, but it's a little hard to find (My Pet Chicken has it). Parakeet grit is also ok. Have fun!
Oh, yeah, the poop question. Poop varies, and a chicken will have varying kinds of poop depending on what she's excreting at the moment, from loose browny liquidy stuff, to almost formed with some white. There's a thread on here somewhere with a link to a good photographic explanation of the types of normal poop, if you search you might be able to find it. If it's just one chick with consistently runny poo, that could be a different issue... I personally wouldn't be too concerned unless the chick were not eating, drinking, active and otherwise apparently healthy.
