simple answer you dont need a different length for TB's
as long as the magnets length makes contact with all the individual slug poles. the screw poles share a metal retainer bar
Yes, I do it. All my guitars are Gibson-designed, and since I buy most of my PU's used, I go for price and sometimes those are on trem-spaced HB's. I swap mags whenever I think they need it, and trem-spaced isn't a consideration.
I saw a scientific article that showed the magnetic field of a humbucker dropping off on the ends, both E strings see less magnetism then A through B. It would be interesting what may happen with a long mag taking up the width of the baseplate.