Looks like a nice bird.
It’s your rocket to do whatever you like, obviously. I am unclear on your decision to do maiden flight on a C to C combo.
For such a small rocket, as you have said will make recovery of both booster and sustainer challenging.
It eyeballs stable, so I don’t see any major safety issues.
I may be in minority, but I feel that every rocket launched should be with intention of safe and undamaged recovery. If those conditions aren’t met, the rocket shouldn’t be launched.
A B6-0 has same max lift off weight as a C, just stages lower. I also believe in using lowest practical sustainer motor for first flight.
Rationale? Staging is cool. I like to see it stage and recover both parts and fly it again. If I succeed I can always fly it again with larger motors.
From a safety issue, priority 1 for motors is getting stack up to speed off the rod or rail. As said, B and C are both adequate, B might be a bit BETTER as it is slightly lighter, so less tail weight, better stability, and with lower mass slightly better acceleration. Anything above this requirement just stages higher, possibly nearly out of sight, which seems pointless to me.
As for sustainer motors, my main excitement is the staging event. Nice to see the sustainer light, make a short trail, after that any extra altitude is wasted on me, just means a longer walk.
For me, if I want altitude I will do a minimum diameter single stage with a bigger motor. Easier, cheaper, and more reliable.
I can see going larger if I am lifting a camera (in which case however I DEFINITELY want it back!). Even then I’d probably start with smaller motors and work up once rocket is “proven.”
Regarding vent holes, aside from taped motors, I always use vents for any gap over say 1.5 inches. But I rarely gap less than 15.0 inches, maxed successfully at around 53 inches. For what looks like your short gap you can probably get away with no vents.
As I said, your rocket, if goal is to lose it then C to C is a good bet.
Straight at trails!