You only need to prune the roots to provide room for them to grow. Roots that become constricted within the pot will hamper the overall growth of the tree.
By cutting off the growing tips on all of the main branches/trunks you force the tree to stop growing up, and cause it to spread out more. However, if the tree is very top-heavy, you might have to prune it quite heavily to encourage lateral branching lower down and it will take quite some time for the tree to reach its original height again, so this might not be the best option for a tree that you want to use in a cham cage (you need the height to allow your cham to get to his basking spot).
Pinching back the leaves at the top would help. By removing about a third of the tree's leaves, you'll not only encourage it to sprout more leaves, but as Jordan mentioned it will also allow more light to penetrate lower down.
I wouldn't worry about the light bulb. The lights are there for your cham, not for the plants. Make sure that the cham gets the right heat and light requirements - the plant is secondary. Most of the time, if the lighting is right for the cham, the plants can cope. At the very worst, if the plants do struggle, you can always rotate sets of plants outside for a couple of months to allow them to recover, whilst others take their place in the cage.
Some people do use additional flourescent bulbs (non-UVB) to help their plants grow, because they don't affect the thermal gradient in the cage significantly.
It's always good to have a back-up tree, especially if you are using Ficus. There is the possibility of massive leaf-drop whenever you disturb a Ficus (sometimes even simply because you move the tree to an area that has different lighting conditions), so it's good to have another tree to fall back on rather than have your cham exposed in a tree with no leaves...
It's definitely worth picking up a book on Bonsai. The basic principle of bonsai is essentially the same as what we are trying to achieve with trees in our cages: to grow a healthy tree within a confined space/shape. There are many bonsai techniques you could use, and most of the trees use for chameleons are also commonly used in bonsai: Ficus Benjamina, Hibiscus, Bougainvillea and Camellia. I've even seen Schefflera bonsai before.