No worries, happy to help

If you have CM6 on your phone now, flashing a new a nightly will NOT erase anything with your phone settings. Apps, ringtones, google account.... will be as you left them after you flash the rom. There is hitch, DO NOT WIPE DATA

You do NOT need to wipe data if you are flashing a nightly. Wipe Davlik cache and cache first, then flash the rom.
Flashing a kernel is the same thing, only need to wipe Davlik cache and cache.
Now, if you decide to flash a new nightly and on your current setup you have one of Deca's kernels, once you flash the NEW nightly, you WILL have the stock kernel again. You will need to flash the kernel again.
2 ways of flashing a new kernel:
1. After flashing the new nightly and letting the phone boot back up, you can reboot back to recovery and flash the kernel again (I recommend this route as I've never ran into boot loops)
or
2. In recovery, flash the new nightly first then flash the new kernel. Then reboot phone