Slots are for upgrades not perks. So even if your slots are locked you can still use perks. If you want to unlock your 2nd & 3rd slot to apply more upgrades. You'll need to level up your account by collecting points (participate in the community hub by giving kudos and answering questions). The 2nd slot will unlock when you reach level 3 (4500 points) and the 3rd at level 5 (15000 points).
If you need more info on how to collect points & level up. Then check this link (https://community.fizz.ca/questions/1826956-rewards-rewards) it might help you understand better.
Once a data perk is applied. any unused data from the perk won't be rolled over to the next month when your payment cycle ends.
Perks can be gifted to and used by people who don't have a data plan.