Is it possible to use the sustain pedal to engage the rotary speed ?

No

Ambiguity.

Is it possible to use a "Sustain Type Pedal" (e.g. Yamaha FC4A) to control the Rotary Effect Speed?
I believe the answer is Yes as per Bill's linked tutorial.

Is it possible to use an FC4A plugged into the Sustain FS Jack to control Rotary Effect Speed?
I think the answer is No as per Jason's reply. I think you need to plug the Pedal into Assignable FS Jack.

Whether I am right or wrong.... it seems there are conflicting opinions to OP's question.

I don't consider something to be a sustain pedal unless it's plugged into the sustain port. A momentary switch - no matter what the original box says it is - plugged into the footswitch input makes this device a footswitch (not a sustain pedal) in context.

So anything plugged into the sustain port can not be used for anything other than sustain. The sustain port is a fixed function port.

Bill's comment is "you can't use sustain, but consider an alternative of the foot switch". In which case, btw, you need to use a latching type switch unless you want to be tied to continually pressing the switch down for one speed.