Depends on the question. Alexander merged with Shiva, and you can use any summon that CANNOT be used by summoner in game.
Shiva, Garuda, Ifrit, Titan, Leviathan, Ramuh, Diabolos, Carbuncle, Odin, and Alexander
Ice, Wind, Fire, Earth, Water, Lightning, Dark, and Light
So that leaves: Caitsith, Bahamut, Quetzlcoatl, Unicorn, Siren, Kujata, etc
Just look at the server names for help.