Holding down the C key is for forcing boot from an optical disc. I have a MacPro with FCS 2 on one disk running SL. Disk 2 is partitioned into 2, with SL. FCS3 on the first partition, Lion and FCPX on the second partition, so three boot options.
To boot from a different disk or partition choose “startup disk” in system preferences or hold down the Alt key when you power on, that will give the option of which disk you would like to boot from.
I have no desire to run an OS which is newer that the software I need to run, it’s just asking for trouble. If it works don’t fix it.