Can you put baby turtles in with adult turtles? I would have to say in my years of experience, no. Turtles will eat or bite anything they can fit in their mouths. The rule for putting your turtles together is: 3 inch difference. If your biggest turtle is 6 inches then your smallest turtle has to be 3 inches or bigger. The second thing you have to think about is how much space do you have. Turtles tend to bite each other if the living space is too small.
Make sure you know your turtle. Many people buy turtles without knowing what the turtle's needs are, and how expensive they will be. The first thing you need to think about is, "Can I afford a turtle?" The second thing after finding out what turtle you want to buy, how big will it get? Turtles do not stay small. There should be 5 gallons of water per inch of shell. (Measure it from head to tail.) The next questions that you need to ask yourself is, "How do they act, What do they like to eat, and what kind of habitat do they need?" In general, turtles like to eat fish, worms, crickets, store bought food, and lettuce. Turtles also act differently depending on what turtle you have or getting. Some are more active than others and some don't like basking often. Different turtles need different homes. Some turtles need shallow water with a lot of hiding and digging space. Then there are others that are hard core swimmers like Sliders in general. Certain turtles need fresh water that stays clean all the time, while others have to have a little dirty water. How you make the dirty water is simple. When you clean the tank, leave an inch or two of dirty water. I know it doesn't sound right, but trust me. It might look dirty when you fill it up, but let your filters do their jobs. Now ask yourself, "Do I know my turtle?"
Check your turtles shell, skin and eyes weekly if not daily. There are several reasons why. Their shell might be soft which means they need more calcium. They could also have shell rot that needs to be treated as soon as possible. Your turtle(s) could have cuts or sores on their skin they need some care. Checking their eyes is easy. Make sure they are not damaged. You turtle(s) should have bright clear eyes. If your turtles eyes are dull and/or cloudy then they need eye drops daily until the problem is gone. Also look for swelling and make sure both can open. Most times the swelling can be cured by the turtle eye drops. If you do not have any medicine to treat any of these thing I have listed, take your turtle to the vet. Trust the professionals, they know what your turtle(s) need. They will also answer any questions you have about turtle care.
Bricks are bad for your turtle. They have dye in them that give them their color. Also, the longer they are in the tank the more they fall apart as little microscopic pieces that your turtle can swallow which is deadly. The microscopic pieces can also float in your turtle's eyes which can lead to as little as irritation and as big as blindness. Do your turtle a favor, don't use brick.