I've been using a Phenom Pro carbon saddle for awhile, as well as an SQLab 611 carbon (both 7x9 carbon railed) on my KS Lev Ci without any issues. The only saddle that's given me any kind of creaking issues is a Fabric Scoop Ultimate, but I think that may be from the saddle itself.
I called KS and asked also. Short answer was yes, just be sure to abide by the torque req. I'm 220 lbs and have been using an Ergon SME3 Carbon Pro 7x9 with KS Lev Ci for about 6 months trouble free. However, the issue I did have was with the length of the head bolts that adjust the seat angle. The 7x9 rails dont leave much excess bolt for nose down adjustment on the rear bolt. With my saddle perfectly level at 0*, the end of the bolt is flush with the nut. My actual seat tube angle is 67°. If I adjust my saddle any more than .5° nose down, I risk not having all the threads in the nut engaged with the bolt. I swapped out the carbon lower portion of the clamp head with the standard aluminum version from the KS Lev because it was just a touch lower profile, maybe 1-2mm. That helped get it to 1° which is my sweet spot. If the rear bolt were 3mm longer I could get about 2° nose down. Its a proprietary bolt also, and KS doesnt make a longer version. So, I would say 67° or steeper (actual, not effective) will work fine.
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