Clustering BP and AP is two whole different worlds all together. It cannot be compared.
Clustering BP is fairly simple.
Clustering small AP motors is much harder than larger ones, because the small nozzle will not allow installation of decent low-current igniters.
Clustering AP is more complicated also because the ignition speed tends to vary with the size of the motor (unless you can use thermite or pellets, what in turns brings us back to point two: Can only be done on larger motors).
Juerg
What Juerg says.
What he is saying will be exemplified and made obvious to you the more you fly (or are around other people who are flying) G+ impulse composite motors.
To illustrate: When you launch a LPR rocket with an Estes motor (really doesn't matter what size) what happens when you press the launch button? Let's assume your batteries on your launcher are in good shape with plenty of charge and your igniters are installed properly.
What happens? Almost instantaneously, the motor ignites and leaps into the air. Focusing on the instant that the motor ignites, the time between "pressing the button" and motor ignition is very small and almost exactly the same every time for a given launching system (again, assuming the batteries are charged and the igniters are good and installed correctly)
Now - think about the G composites. What happens when you "press the button?" Is the time between pressing the button and motor ignition consistent? Hardly.
There are reasons for that, not the least of which is the many variables involved in how long it takes for composite motor pressurization and overall ignition (like: Is the igniter installed in the right place? How much pyrogen is on the igniter? What is the core diameter of the grains? What kind of propellant? What size is the nozzle? Did I bring more cowbell with me today? etc.)
The inherent variablity in ignition times of composite motors make clustering them seriously more complicated than clustering BP motors. Can it be done? Sure. Use ematches and BP pellets (like in CTI Pro38 motors) or use other "exotic" ignition methods other than just regular ol' igniters for example.
Personally, I'd say that clustering G's is *more* complicated than launching an H motor. Probably more complicated than launching an I. Think about that. To fly an H or an I, all you need to do is show up with a rocket that is suitable for launching in that impulse range (LOTS of kits out there in this size. A No Brainer) and build a reload and slap it in there. When actually launching it, you just want to get it off the pad. If it takes a half-second for the motor to pressurize before blasting skyward, who cares, right?
Now, for a clustered G rocket, you need a rocket that is designed for the impulse (now in the H range, see note above - not a problem) and stick more than one motor in there. Reload or single use. If you use reloads, you're adding to the order of magnitude of variability because of the possibility (however small) of user error when assembling the reloads. Anyway, when you get the rocket loaded and to the pad and wired up, you better have made all preparations to ensure that both motors light at the same time for ultimate success. What if one chuffs? What if one spits the igniter and the other pressurizes instantaneously? Hmmmmmmmm....
See my point? If you're flying a single H or I, and it spits the igniter, re-cycle the pad and try again. If one of your motors in a cluster spits and the others don't - Uh, Houston, we have a problem.