Since ReadyBoost caches your RAM to a NAND device, you're not going to need or want to use a USB drive. The storage on the Surface Pro is implemented with a 6Gb/s SSD already, meaning that if ReadyBoost is enabled, you can simply use the main storage device for caching.
If you are having space issues on your storage device, I would recommend a 256GB SD card. You can move some of the data from your SSD drive to the SD card, and then use the SSD for faster caching with ReadyBoost.