Will I be able to have more Crystals and Hearts as I level up?
The maximum amount of Crystals and Hearts does not change as you level up. These values only change when your whole class moves through the class progression — that's when you unlock Hearts!
Guardians, Mages, and Healers have unique strengths and weaknesses, including these stats. It's important for you to work with your team! Guardians have few Crystals but a lot of Hearts, Mages have more Crystals but their Hearts are lower, and Healers are somewhat balanced.