Unless he places you under arrest you may refuse. However, he may be asking you to leave because there is some nearby danger which may harm you in which case you would be a fool to refuse his directions.
If there isn't a valid reason for that police officer to be doing that (e.g., the post is part of an active crime scene), absolutely.
There can be lots, a few are Patrol Officer, Security Guard, WatchMan
a sheriff, body guard, or a security officer(:
Private Investigator, Loss Prevention Officer or Security Guard
yes
Policeman sheriff Fireman Security Guard Correction Officer
a sworn officer wears blue and can arrest criminals while a security guard wears white and can't arrest people
doctor security guard bartender police officer nurse
A railroad may have its own private police force that can be anything from a Security Guard to a fully trained Police Officer. A Railroad Detective is a member fully trained Police Officer that works for the railroad.
Usually yes, however the answer is based on local or department policy so there is no universal answer.
Yes they train you to be a police officer and soldier (kind of)
no, a peace officer in California is a person who has completed POST and is employed by a law enforcement agency, i.e. sheriff or city police. A security guards job is to observe and report, and sometimes protect.