Is it permissible to pray to be reunited with one's deceased fiancé, who died in jihad, in Paradise, and does this supplication reach him?

It appears there is no impediment to supplicating Allah Almighty to unite you with this man in Paradise. The supplication of a Muslim for his Muslim brother benefits him. It is advisable to supplicate that Allah blesses you with a righteous husband with whom you will find happiness in this world and the hereafter.