The plan had been for a peaceful delegation...no one would have known they were soliders until it's too late if Spock hadn't said so....
But the point is - they sent cloaked warbirds along with. If they can send warbirds to Vulcan undetected, wouldn't a much better
plan have been to, you know, do
that, instead of making a great big song and dance about the Romulan's appearance and likely put security on high alert?