Yes, your site servers can be GUPs. Best practice is to install the Symantec Management Agent onto a non-system drive (E:\, for example) so that drives can be expanded or contracted as necessary to accomodate packages. For GUPs, ensure you have planned out the disk space required for the content revisions you're going to maintain, then add that on top of the planned system drive size.
Of course, this is all assuming that you're using configurations that are supported by both products/roles. Since just about anything is supported as a GUP, this really just means ensuring that your site server configuration is a supported one. I wouldn't expand the sharing beyond these two roles, and definitely not a critical service like DNS or Active Directory (AD having known security issues in the configuration).