Can you fly low and slow with the 29mm 29/40-120 RMS? I think so. There is an E16 reload available for it. That's a pretty anemic E motor. On the other end of the spectrum is the G64 that BN150 was talking about.
The other thing that you can do if you put a 29mm motor mount in a rocket is to make a 24mm adapter for it. If you don't want to design one of your own, Aerotech sells a 24mm to 29mm adapter. That way, you can use 24mm motors in a 29mm bird. A lot of Aerotech kits come with a 29mm motor mount (aka mmt) and also have a 24mm adapter included with the kit.
I know that the Cheetah and the Arreaux Aerotech kits come with the 29mm mmt and a 24mm adapter, because I have both of those kits. I'm pretty sure that the Mustang kit has both 24mm mmt and 29mm mmt, too.
Yes, the 24mm Aerotech (aka AT) RMS (Reloadable Motor System) fits in a standard 24mm motor mount.
The only thing you have to be careful with on a 24mm mmt is that there are different length 24mm SU motors. For instance, Estes D impulse motors are 2.75" long, whereas Estes E impulse motors are 3.75" long. If you put a thrust ring in a 24mm mmt at 2.5" for a D impulse motor, then the E impulse hangs out by an inch and a quarter. The thing to do is to either
1 - put no thrust ring in the mmt, and if you use a motor that would normally require a thrust ring wrap enough 1/4" wide masking tape around the nozzle end of the motor to act as a thrust ring
OR
2 - put the thrust ring 3.5" up in the motor tube and then use a 1" adapter (take a burnt D motor and cut 1" of the motor casing and use it as the adapter) when you are going to use a D motor, and leave the adapter out when you are going to use an E motor.
HTH