I had about 4000 hours of Part 121 background, but it was all SIC, plus another 1500 or so in business jets. My TPIC hours were all logged working for an aircraft management company under Part 91 and 135. All the company standards, training programs, etc. were in line with Part 135. But, if I had to guess, I'd say 80-90% of my TPIC hours were all logged under Part 91. They didn't ask for a 91/135 time breakdown in the interview or on the application. Additionally, I was invited to interview before I hit 1000 TPIC. I was pretty close, but I didn't officially cross that 1000 hour mark until after I had been hired and was waiting on a class date.