They'll fit fine. With the single use AT motors, no modifications are necessary, but for the RMS if the mount is built for a standard D motor, you'll need to file down the forward portion of the motor hook in order to allow the delay well of the RMS to clear. If it's an older release (like mine) the motor mount will be designed for the old Estes BP "E" motors, and will include an orange cardboard spacer for use with standard D's. You'll find that the orange spacer fits perfectly over the RMS delay well, and no motor hook modifications will be necessary.
Wow. Did I just post tech?
"Tech" Tod