To answer your last question first, yes, many video production services allow you to mix and match which services you would like to use. Entire video production companies can provide writers, producers, actors (models), directors, art directors, editors, audio engineers, and cameras. Several also have their own studio, sound stage, and media buying teams. They often provide packages for using multiple services to incentivize customers to utilize more services. 

Video productions services cater to a diverse set of clients and, from my experience, can adapt based on your specific needs.