Gladiator is the leader of the Shi'ar Imperial Guard in Marvel.
Mon-El is a DC comics character. A Daxamite which usually is a member of the Legion of Superheroes.
Blackfire is also a character from DC comics. Elder sister of Starfire who is also usually a villain.
I don't know, I would find easier that when Starfire escaped, she and her pursuers got thrown to some portal to the Marvel universe that way no more DC universe characters will be in Marvel universe