All of the instances given in that article were for law enforcement officers. The bit at the top where it mentions security was an umbrella mention for the last part of the article. The only mention it gave to security personnel concerns was at the bottom:
"Under Title 42, U.S.C., Section 1997, the Department of Justice has the ability to initiate civil actions against mental hospitals, retardation facilities, jails, prisons, nursing homes, and juvenile detention facilities when there are allegations of systemic derivations of the constitutional rights of institutionalized persons."
Security guards have no special arrest powers beyond what the average civilian has (Citizen's Arrest). You can hold someone if you see them commit a crime and try to leave the scene. Even then you have to be careful about how you detain the person, Otherwise you can be charged with false arrest. Anyone can do this. However, if you attempt to detain someone against their will you better have other witnesses to the crime. Here's an Article
As far as the use of force goes, it's the same same for security as it is for anyone else. That "failure to keep from harm" clause that you mention pertains to law enforcement only. Security personnel don't take oaths to protect and serve. Police Officers do. Even as a prison guard in two different states, I wasn't asked to put my life in danger to prevent injury to a prisoner if they were doing something dangerous and stupid. Sure, if it didn't put us directly in danger we were to intervene. Otherwise the RRT guys were called in to handle it.
Does it mention what happens to you when you try to keep someone from harm, misjudge the situation due to lack of proper training, and actually cause more harm than you were trying to prevent? That's why security officers aren't asked to be, nor are they considered LEO's. They simply don't have the training that the police do. Period.
If you were to fail to help someone as a security officer, if it made it to court, I would think only the company you work for would have any sort of liability. If the latter situation that I mentioned were to happen you could find yourself and your company being held liable, if not charged outright with a crime.
As a curiosity, what type of security company do you work for? This may
change the situation a bit.