Glad to have your interest! :D

And yes, the spirits can be whatever thing you can think of. If you want to have Quetzalcoatl (The feathered snake) then you can have it. :P And no, using the powers of the spirit won't shorten your life span, since your life force can replenish itself over the course of the night and with a good night's rest, but using too much power at once might drain you dry and you'd actually die.