Make sure to never use pine or cedar -- they are both very bad, especially cedar. They contain toxins called "phenols" and when inhaled, the phenols ruin the lungs and airways. It causes the animal to become quite sick -- you might notice coughing or sneezing or discharge in the eyes and nose. Cedar is the worst because not only does it have more phenols, but it also has little microscopic barbs that rip and tear the airways and lungs and get stuck in there.

My favourite bedding is hemp shavings. They don't smell, they are absorbant, not too dusty. I don't like Carefresh. I find that it smells really bad and I'm constantly finding it around my house. It must get stuck to clothes really easily. Aspen is okay. It can be dusty, and I find that it's odor control and absorbancy isn't very good.

Gerbils are fairly clean. Their poops are very small and they don't pee very much, plus they spend alot of timegrooming themselves. However, you have to make sure you clean the cage often. A friend of mine had gerbils and they would burrow down and pee inside their tunnels. She decided to clean the cage one day. Inside the shavings was a wood tunnel she had bought ages ago. She didn't even know they still had it. Inside the tunnel was sooo much mold and rot, because the gerbils had peed in it. Gerbils don't pick or choose one place to go n, they just go everywhere. One of the easiest small animals to litter train is a rabbit. Rabbits are also good for older children. My eight year old cousin just got a baby Holland lop, and it is the best pet she could ask for. Remember that most cages should be cleaned atleast once a week.

Good luck!

Edit: Oops, forgot to answer your cage question. I know that rats need ventilation, but I hear that gerbils do alright in tanks. My friend just got a really tall tank and filled it with shavings so that the gerbils could burrow.