Judging from the description and your open willingness to break the rules of your job, I don't think you are a good candidate for your position. The fact is that carrying a weapon is considered innapropriate behavior by your company. I am sure carrying a gun on the job is against campany policy and may be illegal as well. The person in question did nothing wrong. In fact, he was doing nothing less than what you yourself were doing. Carrying a knife. He may have been concealing it poorly, or carrying one that is larger than legal, but essentially there is no difference. You are not a cop, or ever a security guard or "loss prevention" employee. It is obvious that there has been little or no training in the legal aspects of your actions.
As for protecting the manager, that is not your job nor is it always going to be possible to protect her in the store due to the size of the store. Having a gun, while you are in another part of the store would not give her any more protection either. If someone were to start slashing with a knife, you would not have time to make it to them in time to do anything but confront the attacker after they have wounded or killed their victim unless you were already nearby which in this instance you were not. Also keep in mind that a gun does not always win over a knife in close quarters. If ther person were trying to steal, pulling the knife or gun would have the possibility escalating the situation, possibly increasing the chances of youself and the person you were hoping to protect to get hurt.
What if the person you described was carrying a handgun? If I were in the store, legally carrying, and was fidgeting with my belt because my gun was uncomfortable and was confronted by you or your manager asking to see what is under my shirt, would you draw if I lifted to show my handgun? Since you asked to see, I would not be doing anything illegal or threatening. You would have no probable cause to draw. If you were to draw, I could sue for your brandishing a weapon, It would involve your store as well since they are responsible for your actions. You may have to pay for your own defense, and would surely be fired. This would make any chance of another job harder to get, may result in the loss of your CCW if you have one. If you didn't have a CCW, this may result in you not getting one in the future either.
I would suggest you need training in the responsible use of a firearm as a civilian and the proper use of force. Being that you are in Califonia, it would seem probable that you would have no chance of escaping prosecution. Before you get the handgun, get a CCW. If you can't get a CCW, realize that almost any situation short of someone already physically attacking you will result in jail time and that you will be facing charges no matter what.
I think best response was stated earlier:
"Duct tape trauma plates to your chest, and maybe shield the manager with your briefcase while your partner assembles and loads the store-issued breakdown NEF single-shot 300 WinMag with an 18" bbl."