Never mind. I was basing this question off of the response data that is received when a command like 'SET_CONFIG' is sent to the API, because it returns the command ID with the response. Since there is no such response data included with request commands like 'GET_VERSION_INFO' it is not possible to safely allow concurrent requests, so I'm going to serialize everything.
As a side note, it would be helpful to add command ID to all responses, no matter what type they are. This way a more robust messaging layer could be built between client apps and your API. However, I will confess that our application doesn't appear to need this at the moment. Serializing the requests should work fine for us.