Do we need to install on separate server, or can we install on the domain controller?
What is your advice please.

No need to install Hosting Controller software on your exchange server. Hosting Controller connects to Exchange remotely via PowerShell commands and therefore no physical installation is required on the Exchange Server itself. You can install HC10 on any of your remote server whether it is a Domain Controller or a Standalone machine for connecting to Exchange and provisioning objects on it via HC panel.
 
Note : We also offer professional support paid service for complete deployment of Exchange, Active Directory etc you can contact sales@hostingcontroller.com to get further help on this turnkey solution by Hosting Controller.

Aun
Do we need to install on separate server, or can we install on the domain controller?
What is your advice please.

What are you requirements regarding HC Hosted Exchange like to number of mailboxes you are willing to go for. Please elaborate so that HC team will guide you they are the best support team.
 
Regards,
Adrian