You can collocate the server roles for Monitoring and Archiving on the same server, but I'm not sure about them both using the same SQL database server instances. For sure if you installed a second SQL Server instance on the same local server as teh OCS roles and used one for Monitoring and the other for the Archiving databases then you'd be fine. It may be possible to store both in the same instance as long as there are not DB naming or other overlaps between roles.
The supported server role collocation table for R2 is located here:
http://technet.microsoft.com/en-us/library/dd425201(office.13).aspx
Jeff Schertz, PointBridge | MVP | MCITP: Enterprise Messaging | MCTS: OCS