Most security guards aren't much anyway. Usually they're the guys with too much phony machismo to take a desk job, but not enough strength to take blue-collar-type jobs that require heavy lifting.
Not that that's always the case--I've seen some SG's who could hold their own. But if the people at the rally stood up to them, the law of averages would probably work in their favor.
A few years back at a local college, the campus leftists held a symposium to discuss how to protest against a planned campus speech by Charlton Heston. A bunch of us pro-2nd-Amendment got the word and decided to attend, arriving in ones and twos to not draw attention.
There were about 2-300 in attendence in the auditorium, with 3 campus rent-a-cops in the back. During the audience-discussion phase, a few of us got the mike, but were cut off. Finally, one of us got the mike and refused to be cut off. The rent-a-cops started forward to eject him. He then yelled "Will the defenders of the 2nd Amendment stand up and be counted?!"
The leftists saw a hundred of us standing -- a full third of the room. The rent-a-cops went back to their donuts. The meeting went downhill for them from there.
There was some silent protest from them outside the auditorium during Heston's speech, but they had gotten the message: when a thousand gun owners show up to hear somebody, DO NOT piss them off