Preening is a very normal behavior for birds. It's how they keep their feathers clean and neat. As long as they are not scratching a LOT or pulling their feathers out excessively it's completely normal.
They will also wallow in anything dusty they can find. All birds love a good dust bath. It is a good idea though to keep feeders and waterers as high as you can. I actually used to put my waterer on a pedestal with a ramp leading up to it so they would get less litter in it.
Chickens by nature will scratch at the ground as part of their normal browsing/feeding behavior, and they tend to kick stuff around, thus the higher feeders and waterers become very necessary.
I always bedded on pine shavings which I simply buried under fresh shavings as they became soiled. In fact, when I would feed them in the morning, I also would dump the leftover remnants in my feeders on the bedding and refill them with fresh food. This allows them to browse around the brooder and "find" food like a chicken would naturally.
Poos will also vary as other posters have said. I would not worry about it unless it goes on for a while.
Also, as other posters have said, use of antibiotics without cause is very bad. I would never medicate a healthy bird as it only causes them to develop resistance to antibiotics that they may need later in life if they actually do get sick.
Just my $.02 Enjoy your chicks and try not to worry so much. They are actually a lot tougher than people give them credit for. They also usually know what they need or like better than we do. lol