I see 2 solutions:
#1: mastering only of one aspect DD/Debuff or Heal/Support so the summoner could excel in one aspect but not both. Letting the SMN choose the way he wants to play and not forced into a role.
#2: ALL summons are sharing the same summoning cooldown, 15 min in your exemple. That would make so the summoner cannot switch summon on a spin and would need to stick with the same summon for at least 15 minutes. That way it would be OK for a SMN to be Excellent with every summon since anyways they would be limited with their summoning Cooldown. The cons for this though that I can see is that if you click the wrong summon in error you are stuck with it for 15min and also I can see people crying that summoner would be able to accomplish anything and fit any role and be excellent at it and cry that is still not fair. (people are very sensitive that way)
You're solutions are bad (false alternatives) and worse (allowing all the summons but giving them a crippling cool down).
Solution 3, only one summon can be equipped or used at a time. Like pokemon, you want to use a different summon, you have to put the first one away.