Summons - How to implement?

So, do we want everyone to be able to summon everything?

Only specific characters can summon?

Specific summons for each character? what?

How should summons work? need more info so i can make the summon system.

Also, should summons just cast a spell like in ff7 or ff9? or should the summon be playable for a specific amount of time? (ff10/13)

Specific ppl have specific summons that match their skills/character