It is not an absolute guarantee that you will not be treated inappropriately, but if you are, you have recourse to file a grievance. Union actors also get better wages and better working conditions than non-union actors. It is almost impossible to build a decent career if you are not a union worker. So never sign with a non-franchised agent. Before you sign with any agent, call the local SAG office to make sure that the agent is still franchised.
Commercial agents handle commercials.
Theatrical agents handle movies and television. Legitimate agents handle theater. Modeling agents handle print and modeling work, variety agents handle nightclub and personal appearance work, and full-service agents handle all or some of the various types of acting work. If you don't have much experience, you may want to try a commercial agent first to get some experience, or both a theatrical and a commercial agent, or a full-service agent who can handle television, movies and commercials. And you'll have a much better chance to be seen if you do commercials.