I believe all standard fiends are in there regardless. Certain bosses (Chocobo Eater, Jumbo Cactuar, King Vermin, and Azi Dahaka) only appear AFTER you've beaten them. If you're following a 100% guide, you'll encounter all Except Azi Dahaka.
Even if standard fiends are only in there after you defeat them, Almost all fiends can be encountered in the Creature Creator, with a few exceptions.
It should be mentioned that all Oversouled Fiends in the game can also be encountered in the Via Infinto
Also there is a trick forcing to get a specific Oversouled creature.
1) You must encounter the "type" of fiend who is already in Oversoul status. Meaning getting into a battle and making a monster go into oversoul. If you are looking for a Flan, then you must encounter any Flan-type to go Oversoul.
2) Once your prefered monster type has gone Oversoul, flee the battle without killing the monster.
3) Go to the area of the desired "type" monster you want to add it's entry into the Bestairy, and then kill it as it should automatically enter Oversoul status once encountered.
This trick works for all fiends that can become Oversouled. Using Weapon-fiends for example, i.e Ultima, Omega, and Paragon.
For example encountering Ultima or Omega Weapon and them going Oversoul, you can flee the battle, then get into a fight with Paragon and he will Oversoul.
This is good way cut down a lot of time getting the Oversouled fiends into the bestiary.