You could buy one that's not on the list and try it but if you find one that IS on the list then you can buy with confidence. Most flash drives will be recognized right away when you plug them into the Xbox. You would plug it into one of the ports (or a controller memory unit) and the Xbox will come up with a message saying "your memory unit is damaged" or something like that. Then after you get out of that message it should be good to go to use as a memory unit. If it doesn't work right away try a low level format on your PC. If that doesn't work it may just be incompatible.
Keep in mind if you would like to use the flash drive memory unit on your PC, you'll need memory unit drivers.
You can get them here: Drivers.zip