With the KDE 600XF motors, you can adjust the amount of shaft slop with the brass collar. My understanding is that you do not want visual slop, but want "some" up/down slop, where you just hear a "click" when jiggling the motor shaft up/down.
As far as the pinion to pinion support bearing relation, it is my understanding that helical pinions/gears benefit from the pinion lightly resting against the pinion support bearing, mainly to minimize axial stress on the motor bearings.
This is the reason Patrick sites in the KDE website why he does not recommend using helical gears on the T-Rex 550, since it does not use a pinion support bearing.