Yes, you can do this, except with Tier 4 students – you must add the number of hours you are specifically commissioning them each week to ensure that they are not over commissioned vis a vis the hours permitted by their visa. 

You are unable to submit zero hours for any assignment. 

You can and must adjust future assignments if the worker is trying to input hours more than their weekly assignment hours.  Please adjust the weekly, total, and minimum hours if required.

When creating assignments best practice is for the assignment to start on a Monday and end on Sunday, even when the assignment is only for a few hours / days during that duration.  This will avoid overlapping of assignment and make it clearer for you to view as a Hiring Manager.