This may be a simple question to post and i believe that i know the answer i just want to confirm what i believe. Can you install Mobile Admin Server on a different server, for example a Web server within your DMZ as this will be internet facing, and then link it back to your main Orion core server?
If so what ports does it use so i can open them up in the firewall?
Is there also a Admin guide for mobile admin as i could only find an old version from the rovit website?
Yes, many customers setup Mobile Admin in this exact fashion. Mobile Admin will sit in your DMZ and then you can configure the MA Server to point at your Orion Server. To do this, you'll need to open ports 17778 and 17777. I've linked to the relevant section in the Admin Guide here SolarWinds - Rove Mobile Admin Documentation
The Admin Guide on RoveIT.com is the current version of the documentation. It is fully up to date.