Question regarding sixth sense.

Dakiras: Perks are only available at 100% as you rightly have said, so it's actually advised to take something else first on the commander. Something like Camo, repairs, or whatever you like (depends on the tank) and then when the first skill is at 100% you drop it and retrain the perk. This way you're not grinding from 0-100% with an inactive perk. :)