Welcome to SatelliteGuys. You are off to a great start and have accomplished the hardest part of any install, tracking the arc with the motorized dish! Congrats!
Like KE4EST sugggests, you might catch a few C-band signals with a 90cm, but only the strongest like LESEA on 99w. Even then, everything will need to be optimized to lock even the strongest C-band transponder.
1. When the C/KU feedhorn is installed, the mount may have raised the center of the feedhorn above the center of the previous KU LNBF, moved the feedhorn opening closer/further away from the reflector or changed where the feedhorn is pointed into the reflector. Reinstall the KU LNBF aim at 99w and take note of an active and working transponder (Example: 11800 H 30000). Make a template of the position and aiming of the OEM mounted LNBF. Now install the C-band clamp so it can position the C/KU feedhorn opening in the exact position and angle.
The C-band LNBF output is the forward port and the KU is the back port in the orange plastic cover. You are correct that the WSI C/KU's KU LNBF is Universal LO and typically is not used in conjunction with a 22KHz switch due to the potential conflicts. The seller should have included a DiSEqC switch and not a 22KHz type. You can make a 22KHz switch work by connecting the 22KHz port to the KU port, but this might be confusing when first starting out. Let's first connect straight to the KU section without the switch inline. Set the install menu to 99w KU and the LNBF type to Universal and the LO setting to 9750/10600. This will grey out the 22KHz switch setting as it becomes automatic when Universal LNB type is selected. Do not install the dielectric slab in the feedhorn throat, as this only is useful for C-band circular polarity signals and there are none on 97w. Do not install the scalar at this point. Select the active transponder and verify that the Signal Quality reading is good and high as with the previous KU LNBF. If not, optimize by adjusting the feedhorn forward and aft and slight adjustments to the skew.
Once KU-band is coming in on 99w, it is time to check C-band. Set the Install menu to 99w C-band, the LNB type to Single (or Standard) with LO frequency of 5150. Select the LESEA transponder (4000 H 26400). The transponder should be giving some Quality meter reading. If not, try adjusting the feedhorn forward and aft and slight adjustments to the skew. Once Signal Quality reading is optimized, add the conical scalar and position to optimize the Signal Quality reading. Now adjust the feedhorn forward and aft and slight adjustments to the skew, then the scalar again. Repeat between the feedhorn and the scalar adjustment until the Signal Quality reading is peaked.
2. Doubt that a 100cm will give much more C-band than with a 90cm. Probably not worth the extra cost with minimal gain. It will still only receive the absolute strongest transponders. I would suggest that the minimum size for a microBUD is a 1.2m as it will provide more gain and a few more transponders.
3. If using the 22K switch, the C-band LNBF will be connected to 22KHz OFF port and the KU sLNBF will be connected to the 22KHz ON port. To trigger the switch for KU operation the KU LNB type is set to Single (or standard) with LO setting of 10600 and the 22KHz menu setting to ON. To trigger the switch for C-band, the 22KHz menus setting will be set to OFF.