The surplus 1/4" shaft coupling works perfectly on the slow motion control shaft of the TG-SP II, of course, it's too big for the small geared motors.
So now I've two choices:
1. buy another small shaft coupling with 3mm bore and with OD ~ 1/4 inch OR
2. buy another set of geared motors with ~ 1/4 inch shaft
I guess I prefer (2) since the small motors which I got now could be a bit too fast (5 RPM at 6V) and maybe too small to deliver enough torque. And these smaller motors can be left for other projects like motorizing different focusers.