Fresh fruits and veggies diced up is the best treat you can give them. also meal worms, small amounts of yogurt, warm oatmeal mixed with fruits, cooked pasta, in small amounts, or small amounts of cheese.
x2 Around 20 weeks I would add some oyster shell on the side. Once all your girls are laying you can decide if you want to switch to a layer or stay grower + calcium. Either is fine depending on your flock and your goals.