I don't quite understand why Ifrits wouldn't fit in with the demons; they fit in the 3rd part, and as for the backstory... Forget about it; we can come up with a new one for the 6th part.
And again, you didn't understand what I meant about the number of creatures. I mean that in the castle, there are 5th-level creatures, with 2 units for each level (a total of 10), and all of them can be built and recruited, while the boss could have 8 or 10 slots for his army.