Forget about the C/Ku combo. Those are a compromise between functionality and performance. Since using a 90cm dish for C-band is already way smaller than what is normally recommended, you can't afford to waste anything more in terms of signal quality.
I have tried a C-band LNBF with conical feedhorn on a Fortec 90cm and could not get much. The only channels I was getting somewhat reliably were The Word and Classic Arts Showcase on 91W, and a few very pixelated channels on 40.5W and 113W. I was also limited by the receiver i had at the time (DVB-S only) so there might be a few more. The LESEA channels on 99W are usually an easy target, and so is NHK on 58W, but I was not getting those.
In the end, it was fun to try it, but a bit frustrating. I don't want to discourage you, I don't regret trying, just don't expect much from it.
Since then I was able to find a used 10-foot dish and my LNBF is getting very good use.