So if I was to go only by the recommended engines in the instructions, which I'd rather do, go with the max of D12-0 in booster/2nd stage and C11-5 or D12-5 for sustainer/1st stage?
Yes... depending on the size of the field and the conditions at launch, and the size/expected altitude of the rocket...
The first stage (booster) motor generally needs to be bigger, because it's lifting the weight of the first stage and the motor itself, but also the upper stage and ITS motor, plus the recovery device, and any payload (if applicable). All this extra weight (particularly of the upper stage motor, which is usually the heaviest component) plus the extra drag created by the upper stage fins and airframe means the first stage booster motor must have a pretty good 'kick' to get the rocket up and moving off the pad and accelerated to a sufficient velocity to 1) be stable by the time it leaves the launch rod (accelerated to approximately 30 mph) and 2) have sufficient airspeed and altitude to remain pointed upright and stable during the short interval (usually part of a second) for the first stage motor to burn out and the upper stage (sustainer) motor to actually ignite, come up to pressure, and separate the stages and continue accelerating the upper sustainer stage away from the booster during the continuation of the flight.
Since the rocket is already up and moving at a considerable velocity at staging, the velocity of the first stage adds to the velocity imparted by the upper sustainer stage's motor, so the rocket reaches a considerably higher altitude than it would with the same motor lifting the upper sustainer stage from a standstill off the pad... The higher the altitude, the more drift one can expect during the recovery phase due to wind, and therefore the farther downrange the rocket will travel before landing. Therefore, you need to be aware of 1) the flying conditions when you're launching (high winds and high altitudes= lost rockets a lot of times) and 2) the size/surroundings of the launch field. Using a smaller upper sustainer stage motor will reduce the maximum altitude the rocket achieves and thus should keep the rocket landing closer to the launch site, and hopefully within the flying field. If you have high winds on the day your launching, or your flying site is surrounded by rocket-eating trees, neighborhoods, water, other private property that might make recovery difficult or impossible if it lands off-field, etc. then you want to choose the smallest recommended motors for the booster first stage and sustainer second stage that the instructions recommend. If it's a calm day, flying from a LARGE field with plenty of safe, open, downrange recovery area for the rocket's upper sustainer stage to land in, and you want to go for the absolute maximum altitude/performance the rocket can achieve, then go for it and use the largest recommended motors!
Later and hope this helps...
OL JR