Q: Mac Pro 2013 and Windows 7

Bootcamp Assistant (BCA) states it won't take anything earlier than Windows 8.

I thought it was "8.1"

I know Windows will boot from Thunderbolt, but has to be installed on internal SSD, then cloned to Thunderbolt, and you can reclaim the space BCA set aside later.

I would have to look, there were excellent threads on MacRumors going back to early 2014 on how and if you could skip doing the install, and just partition and download the drivers.

Do you want UEFI or not?

I have more trust in avoiding BCA and doing a "native" unassisted install

http://forums.macrumors.com/showthread.php?t=1832017

http://forums.macrumors.com/showthread.php?t=1653997

Just sharing my experience on this thread - I was able to use the instructions here - Install Windows 7 or 8 on an external USB3 or Thunderbolt drive without using bootcamp and used the StarTech Thunderbolt Dual Hard Drive Enclosure. The enclosure has two SSDs, one dedicated for Windows and the other that's my storage drive for OSX. I can boot off the enclosure by holding the option key and selecting the Windows drive. Seems to be working fine.

http://forums.macrumors.com/showthread.php?t=1692097

Mac: Install Windows 7 or 8 on an external USB3 or Thunderbolt drive without using bootcamp

I could not find the exact thread I was looking for to do exactly what you purpose and want.

http://forums.macrumors.com/showthread.php?t=1692097

According to Boot Camp: System requirements for Microsoft Windows operating systems - Apple Support

W7 64-bit is supported, but W7 32-bit is not. Are you certain your W7 media is correct and complete? If the media has both 32-bit and 64-bit on it, it will be rejected.