I'm using an Iscomorphot 8 1.5x with 5D Mk II. Your question on which taking lens is suitable is something I have been trying to figure out and experimenting with, to the detriment of my wallet. Try reading Andrew's anamorphic shooting guide for clues. My personal findings (may be wrong) are that to minimize vignetting on a full frame:
1) If you are not using crop mode, don't go shorter that 85mm for taking lens (I would guess 105mm is a safer bet)
2) Choose a taking lens that has a very narrow lens barrel (pardon if I get the term incorrect), meaning the lens diameter is not fat like a Sigma 35mm F1.4, but very thin like the Asahi Pentax 35mm F3.5 (M42) that I am using (risking daily the mirror hitting the rear element of the lens)
3) Try using crop mode paired with a wide lens (35mm x3 will be 105mm) - I searched ebay and found that even old manual focus lens in the 85mm-105mm focal length are not "thin" enough as mentioned in (2) and thus non-crop mode will be a struggle.
4) Mount the anamorphic lens as close as possible to the taking lens front element.
Hope that helps.