"Answer hazy, ask later." :-)

The KSM55 is a new one on me, but the service website brings up the owner's manual for it which does also include the KSM60 and KSM70; and the service information refers to the old "commercial" architecture machines. (*)

It looks like it's a badge-engineered version of the KSM70, with a smaller bowl and less fancy accessories.

It should move very little on the countertop; when kneading heavy dough or large batches, make sure you keep the speeds low (speed "2" or below).

(* Speculation: it's possible that KitchenAid may be phasing out the old "Professional" design. This is decidedly mixed news: the Pro machines had a flawed design for the planetary which is addressed in the new design; but the new design has an integrated motor/transmission unit that isn't field-repairable and can only be replaced as a whole.)

The professional 500 600 and 700 models are all being discontinued. The 5.5 qt is a new model. The motor is dropped in differently (and doesnt have a visible black screw). The wattage is the 500 (not 525) so less than some of the older ones but that shouldn't be noticeable.