you will want to store the GarageBand project you are currently working on, on your fastest drive. I store my current projects on my internal SSD. When I am finished I move them to an external drive.
time capsule (external drive)
Your TimeCapsule is connected by network, so it probably will be slower than working with GarageBand on your internal drive. Just try it.
Also is it possible to just store data of the Time Capsule ext drive without screwing up the time machine utility?
TimeMachine will want to use the full drive. What some people are doing, is to put a writable disk image of fixed size there and mount it, when they want to write there. But it would be less trouble to connect an external drive to your TimeCapsule.
You may want to have a look at Pondini's excellent FAQ for all Time Machine questions: