You have ample freedom to manage your translation center. 

Regarding clients, for instance, you can have all jobs created by your own team on behalf of the clients without involving the clients in any way.

You could also have an expert from the client actively involved in one or more jobs, and interacting directly with the translators and PM, while keeping the isolation in all other jobs.

You could also give one or more clients access to post small jobs directly in your platform, so they can be managed quickly and with a light overhead while you keep your complete procedures for the larger projects.