Can somebody provide me a link to the CIC official website, stating that one cannot qualify under FSWP if they only TA/RA work experience?
CEC is explicit in that work experience gained during full-time study will not be accepted. You also won't gain any points under Express Entry if your work experience is gained while you are studying full-time, but there is nothing (I repeat, nothing), stopping you to apply under EE and qualify under FSWP if your work experience is only TA/RA.
Yes, you will be at disadvantage since you won't have any work experience points under CRS, but your work experience will be accepted under FSWP, and if you can get at least 67 poitns under FSWP, you should technically quallify to Express Entry (again, with no work experience points under CRS, but still qualify nonetheless.
For those wondering if you can get hours on your work experience, yes, it is possible. I got my PR under the old PhD stream, and I got my reference letter from my supervisor. I too didn't have any other work experience but TA/RA at the time (keep also in mind that most of my funding was non-taxable, like most other PhD students), and yet, I STILL GOT MY PR.
Make sure your letter follows the exact same format as required by CIC. Keep in mind that you will have to ask for it. The boiler plate letter HR provides won't do!!! Document your hours properly, and use the correct job descriptions. A lot of people were rejected because their job descriptions did not match NOC 4012.
Again, anyone is welcomed to point out the specific quote for FSWP that precludes TA/RA from qualifying. It is explicit under CEC and CRS, so if it was really the case, they should mention it under FSWP as well (but they don't).