...explaining that we only have about 300 clients in each site that each patch manager server will responsible for,
How many sites/clients total in the enterprise?
I'm assuming from the description that these sites already have WSUS servers. If so, then yes, it would be appropriate to install a Patch Manager Automation Role server on each replica WSUS server.
Would it, in fact, be acceptable (performance wise) to install Patch Manager roles (all three) on the same 2012 R2 servers we are running WSUS from if there are at maximum 300 clients that will connect to each?
In reality, unless there are significantly more sites/clients than I'm imaging, you would likely only have one server with the Application Role and Management Role. Remote sites would have only the Automation Role.
Can Patch Manager be installed to a Windows Internal Database,
No. The WID is licensed only for use by Microsoft-native applications.
- Install SQL Express (2008 or 2012 - whichever is supported) and install Patch Manager's database into that and leave WSUS running on Windows Internal Database.
- Install SQL Express (2008 or 2012 - whichever is supported) and install both the WSUS and Patch Manager databases into that.
This really depends on a number of additional factors.
- Where do you plan to install the Patch Manager Primary Application Server?
- How many clients, total, in the enterprise will be managed by the WSUS environment?
- Will you be using WSUS Inventory/Reporting via Patch Manager
- Will you be using the Patch Manager WMI-based Managed Computer Inventory functionality?