Stiles could simply be mistaken, of course - after all, only one ship was ever seen in action. A more objective analysis at a later date might indicate the likelihood of further ships on the Grassy Knoll.
Possible, but let's take a look of the Romulans take of their mission:
DECIUS: Only in code, Commander. To inform our Praetor of this glorious mission.
COMMANDER: Your carelessness might have ended this "glorious" mission. You're reduced two steps in rank. Return to post.
COMMANDER: I think you do. No need to tell you what happens when we reach home with proof of the Earthmen's weakness. And we will have proof. The Earth commander will follow. He must. When he attacks, we will destroy him. Our gift to the homeland, another war.
Had there been other ships involved, would have every ship claimed a "glorious mission"? It sounded like a unique "achievement" to me. And it appears that a new war solely depended on what this particular Romulan ship did or failed to do next.
I don't stay with the idea of the Romulans being confined to a single star system.
And you don't have to, The Making of Star Trek
(page 256) is very specific on the matter: "While little is known of the Empire as a whole, we do know that it encompasses several solar systems."
It then would appear that "ROMII" is either the star system "Romii" or "Romulus II".