irDigital, you will find two things.
TM will eat drive capacity until none is left for other "stuff"
It does not make much sense to backup one partition of a drive to another partition on the same drive as one disk failure and you lose both your data and its backup. So don't bother the partition approach, been there done that.
For you as well as most, if you need an external drive for "stuff" you need two physical drives (they are cheap enough currently). One for the stuff and another larger one to backup both your internal drive and stuff drive.
Crashplan works for some, well enough if all you have are a few files, but is slow and really sucked for my purposes. Not only that, the crashplan software corrupted my system to the point that I had to do a fresh OS install. The net is full of similar experiences. Thanks a lot crashplan. I would stay away, especially if you need to backup any files more than ~5GB in size.