The stable could have slots that you can't summon demons from, but you could swap demons out of these slots. Doing this, however, sets the demon's HP to 1 and its SP to 0, so you can't use them right away(outside of cheeky strategies). There are a few variables for implementing this:
-How many inactive slots does the player start with, and will the number of active slots be reduced to compensate?
-Will there be upgrades that add more inactive slots, or perhaps the current capacity ups will add them as a bonus?
-Do enemy summoners turn into even more oversized experience pinatas?