In my opinion, the two main purposes of references are to 1) build your file for a defense that you believed the provider was credible and 2) present a provider to a client that they will accept. So, the short answer to what is acceptable is would the references you have on file help defend why you presented that provider to a client and are those references going to be accepted by the Client when they undergo Credentialing. The parameters for the above can depend on the agency. But, some of the generally accepted considerations are:
What is an acceptable reference for a Provider?