Question regarding Quintessential Kobold feat: Knee Biter


Hi :)

The feat reads:

KNEE BITER [General, Fighter]
You can use your small size to target vulnerable areas in a fight.
Prerequisites: Base attack bonus +1, Small size.
Benefit: When you deal a critical hit your opponent must make a Fortitude Save (DC 10 + the damage dealt) or lose 5 feet from his movement rate until he is magically healed or a companion makes a Heal check (DC 20) to repair the damage.
Normal: Critical hits do increased damage.

* does the loss of movement rate replace crit damage or is it with crit damage?
* does loss of movement rate stack? For instance, if a foe has been hit with critical hits three times and failed three saves, has its movement rate been decreased by 15 feet?

Thanks :)