I heard that a siyum during shvua shechal bo is different than otherwise during the 9 days.
saw this in a daily halacha email i get:
"Once the actual week of Tisha B'Av arrives, only ten of the guests (besides the ones making the Simcha) may eat meat and drink wine, while the rest of the attendees must eat non-meat and wine items. (Rama ibid. and Mishna Berura S"K 77)"