Although I'm not a lawyer, nor have I ever hired security before, I have worked security in both low and high risk areas as well as been a police officer who worked off-duty security. Also keep in mind that this is dated information from the 80's and mid 90's.
From what I have observed over the years, businesses hire security for insurance purposes only, and the guards are there to observe and report as a deterence only. Armed guards increase liability which is why many security companies do not offer armed guards, but if they do that cost is billed to the client.
If I were going to hire a guard I'd go with an established security company or hire off-duty LEOs keeping in mind that LEOs sometimes forget that they are working security rather than law-enforcement.