Regarding your original question:
I did a lot of talent shows and variety shows in the high school where I taught. These were usually put together by other organizations and the theatre department provided the technical support ( i.e., the SM, ASM, running crew, lighting and audio). My SMs were mature students, usually veterans of the department, and had at a minimum already SMed a musical.
Generally, I found it very useful to sit down with the head of the project (I'll use the term "director" for ease of typing) in advance and explain the SM's role to him or her, emphasizing that the SM, although a student, had absolute authority backstage, including the authority (1) to remove any performer from the show if that individual became problematic onstage or backstage, and (2) to change the order of the show if technical difficulties required it. This was a no-bargaining point for me if the director wanted any tech support. The director was responsible for emphasizing to the performers. That way, when I met with them, it was reiteration and not new information.
Another thing that was enormously helpful was technical needs forms. I created a form which the performers had to fill out in advance that gathered all the information I would need about each performance--from the track number on the CD accompaniment, to the number of electical outlets needed, to a description fo the action that was going to occur during the act). Since most people have no clue what it takes to pull off a successful variety show, this accomplished several things:
(1) It forced the performers to make decisions about what they were going to do and commit to their decisions in writing. This also had the effect of assuring the performers that we wanted their performances to be successful, while at the same time further establishing us as the authority as the ones running the show.
(2) It let the performers know the limitations within which they had to work--a max of three wireless handheld mics, for example.
(3) It gave us an advanced heads-up on potential technical problems (stage lights in the baton-twirlers eyes), safety issues ("No, you may not throw a bucket of water on stage or scatter confetti on the stage right before the next dance routine!"), difficult shifts (striking the scenery from the skit while simultaneously setting up for the 6-piece band--in 20 seconds), etc.
(4) It heped us have sufficient equipment on hand and pre-arrange the order of the events in the show with the director to maintain the aesthetics of the show without creating impossible shifts or long delays between acts.
If they are not well organized IN ADVANCE, rehearsals for variety shows inevitably become ridiculously long and frustrating, and can get out of hand very quickly. But if you do your job well up front, you will earn enormous respect from the director and the performers when they realize that their show looked polished and professional because of you and your efforts.
A final note, although it may seem pedantic: You must have excellent headset communication among your crew members to do this kind of show. There are often lots of spontaneous decisions to be made and everyone has to be on board to execute them smoothly.