No.
Fortress Maximus is just like other character cards with a body, except it's bigger, and it must have Cerebros as its head. Cerebros is unique in that it has both a head mode and a body mode. So it starts the game attached to Fortress Maximus, but you also get another head to put onto Cerebros. This can be Emissary or any other character with a head mode.
The stars from all three cards count toward the 25-star limit. Note that Fortress Maximus, Cerebros, and Emissary have 25 stars combined.
Can I play Emissary with any character that has a body mode?
[Source: WotC]
Yes. Emissary is just like other character cards that have a head mode and a bot mode.
No. You may use any 4 star or less Head mode character.