I'm afraid there isn't a specific "rule" that guides this. It's more of a conglomeration of business decisions and making sure you don't break other rules that guide this answer.
Technically, I suppose a company could have multiple fee schedules - but then you would have to monitor them frequently for compliance with your contracts. For instance, you couldn't charge anyone else less than you charge Medicare patients. And if you have specific contracts, you'll need to make sure your contracts don't prohibit you from having multiple schedules or any other binding issues.
It is much easier, from pure business sense, to have a single fee schedule that is charged across the board and then different write-off adjustments to meet the specific contract requirements. This allows you to show consistencey, defend equal treatment (show that you aren't giving prefential treatment to any group), and justify discounts or reductions.
That's the quick and dirty answer. I know it doesn't give you the specific something 'in writing' you are looking for - but at least gives you some things to consider.
Melissa Brown, RHIA, CPC, CPC-I, CFPC
Past Chair AAPC Chapter Association Board of Dirctors
Past-President, River City AAPC
Executive Co-Chair, Coding on the River