Skp;
6 canted MMX-II or MMX-II-NE (sort of boosters) is not out of the question. While I haven't built a tractor Micro cluster, I've built and flown many 6, 8 & 10 motor standard cluster micro models including a 6- inline XB-70 pmc (Super Heavy)
You are absoluely correct you will need pyrogen tipped igniters (supplied Q2's with a pyrodex-NL dip works well.) and a RELAY ignition system with the HD battery at the launcher.
Keep in mind no matter how well made a clip-whip, it is ONLY an extension of the Launch system to which it is connected to. Your clips leads should be kept as short as practical for you application (try not to exceed 18-20") and should be at least 16ga
Stranded copper wire. while connecting these leads to igniters try to leave a small loop of wire that will allow the model to begin to raise before ripping out the igniter.
I've successfully flown 6 motor MMX clusters from a 10amp DPDT relay (it is important to break both Pos & Neg poles) using a freshly charged 7amp/hr gel-cell battery.
My only concern with this project is the SIZE and expected Mass of your Guitar. I also play the guitar so have a pretty good idea of general size. You haven't mentioned what
Scale you will be building this Odd-Roc. Could it be a ukulele instead of a full size guitar? Are you planning on down-scaling the guitar model to keep the model somewhere around 42g to 85g??
MMX-II and MMX-II-NE motors are .285Ns motors both have an average .854s delay but the NE motors have no ejection charge (used as Booster motors). MMX-II's initial weight is 1.1g, MMX-II-NE initial weight is 1.0g. Perhaps these numbers will help a little. 6 MMX motors will give 1.71Ns of thrust (a very low end A motor) so if it were my model I'd try to keep the LOWt. around 42-50g to give it any chance of somewhat decent altitude (the XB-70 gets about 12feet).
something else to bare in mind; by canting the motors we are also reducing the applied thrust, meaning the model should be built on the lighter side.
Good luck, Can't wait to see a drawing or photos of the finished Odd-Roc.