A safe way to get Rivet City Security Uniform?

The easiest way to do this is to engage in a trick known as Reverse Pickpocketing. In short, what you need to do is plant a higher quality suit of armor on the individual wearing the gear you want (measured by defense rating). They'll then equip this armor, leaving you free to pickpocket the gear they're wearing. The Metal Armor and Helmet are a good choice for this, because of their high DR value, and relative low cost. Other options include any other Combat Armor variant other than the Security Uniform, or of course, any Power Armor.

An alternative is to make use of The Mesmetron to stun/hypnotize a guard and demand they hand over their stuff. This may end violently however.

If you haven't yet reached Max Level, picking up the Mr. Sandman perk will allow you to slaughter sleeping Rivet City Guards left and right, as long as you please. No one will be the wiser.

If you've got Broken Steel, then after you recover, there are rivet city guards defending the memorial against Enclave, and they often die.

If you fast travel there, you can watch, and pick up the pieces afterwards.