Alexander would be pretty hard to do for a battle like the other primals. He is more like Bahamut. He will be involved in the story, but it would be hard to call him a primal battle, especially since he is not evil or dangerous.

Looking at the summons from FFIV and FFVI would be best, as Rydia collects a lot of them and there is an Esper village in VI. FFVII though has majority of all the primals already in game.