Well, then I will fill what I known so far.
Rank 7 Fleet Mission have a chance of spawning:
Niagara, Cerberus, Renommee, Surprise, Frigate, Belle Poule and Trincomalee.
Rank 8 Solo Mission (only tried this once):
Pavel x1
Rank 8 Fleet Mission have chance of spawning:
Constitution, Ingermanland, 3rd rate, Bellona(?), Pavel
That all I known since after the update. I assume that what ever you meet at fleet order you will have chance meet at solo order (this is my theory, not tested) although number is unknown. Fleet order is good enough for a fleet to handle, I have seen from 6 to 10-ish enemy ship so far.
I think you might got drag into a event. If there is AI with your team, then its a fleet order, if not its one of those epic event