I think the energy ball should fill whenever you increase a level in gardening skill, and maybe also when you rank up a pet. I think the gardening level idea should be implemented for sure and am not quite sure on the pet idea.
I think that all your pet/gardening energy should refill when you level up your PET. BUT when YOUR WIZARD levels up all the health, mana, and pet/gardening energy should refill too. Just a suggestion though? Please change this on wizard though.