Try disconnecting all of your internal hard drives first, then set up XP on the flash drive. This way it won't mess up bootmgr. You will have to go into 7 and manually add an entry for XP later though.
I had thought about that, it should also work if I turned off all of my internal drives in the BIOS as well, correct? I'll have to give it a try. Do you know of any good guides on editing the bootmgr after the fact if I can get it working on the flash drive first?