OK, so here's the good, the bad, and the moderately ugly.
The good:
* The 18mm sustainer fit perfectly into the ID of the 24mm booster. I knew that they nominally fit, but I didn't realize how ideal a fit it is. That meant it was easy to keep things collinear.
* Stability was good for the full stack. Nice boost off the pad.
* The sustainer was surprisingly stable under spin. Even though it had half a caliber of negative stability, it flew in a somewhat wide spiral rather than tumbling.
* The booster tumbled nicely on return to earth. It was maybe a little faster than I'd like, but fine for a grass field.
The bad:
* I need to figure out print tolerances better so that I have better control over the fits. The booster to sustainer joint was far too loose on just the motor and a bit tight with the body tube. I had to jam it in fairly tight, which led to a couple of other issues.
* It took a lot of tape to friction fit the booster motor in. Even with that plus a couple of wraps of electrical tape on the outside, the booster spit the spent motor casing. The electrical tape broke clean away. That might have impacted how much burn damage there was to the booster.
The ugly:
* The moment you're all waiting for: pretty serious burn damage to the booster. The stuff inside the body tube isn't really surprising, given that the booster spit the spent casing. The damage on the fins and inside the ring did surprise me. The fin damage probably isn't enough to keep it from flying again, but the burn inside the tube looks like it goes partway into the structure. As previously mentioned, this is overbuilt, but...
Next steps:
I like the concept, so I think I will replace the booster. I'll reprint it with thinner walls and fins, and adjust the tolerances of the tubes so that the sustainer fits snug but not tight in the upper part and I can line the lower part with a piece of 24mm body tube. I might leave clearance to line the sustainer mount with tin foil or can as mentioned above. I'll definitely print in the launch lugs too. Painting the outside with grill paint seems to be in order. I do have aluminum tape for the outside, which would be nice and flashy.