This is neat stuff. I'm a fan of FF's summoners so I too hope they make it in. In regards to the way you summon spirits, to promote job uniqueness I could see the spirit version being made available to the disciple of magic EVK, (pet elementals) that are generic, and to summon their more potent counterparts like Light Spirit > Carbuncle, Earth Spirit > Golem, you would need to be playing SMN. SMN would still have access to the lesser spirits if they so chose for MP conservation/efficiency and such.

The method of equipping idols that you suggested could turn into an inventory nightmare if it is indeed bound to the inventory itself, so I would suggest making a new slot for a key item that works similarly to job stones.