is entirely different that 6.9 DS. When you install SMP, and then setup site servers, you are extending the ability of the platform to 'reach' clients without affecting the WAN. Site Servers also take pressue off the SMP server itself.
You can add a site server from the site server configuration screen, and enable the task and\or package services. If you want to extend the ability to use DS7.1 on these site servers, you have to enable the component separately.
If you want to use DS6.9 instead of DS7.1, you can do that, however it won't be integrated into the SMP platform. You could make the DS6.9 a site server, but it still wouldn't be part of the DS7.1 framework without the components I mentioned above.
Your first step is to consider what you want to use for imaging. If you're going to to stick with DS6.9 and perhaps only test DS7.1, I don't know if you'd even want DS7.1 installed on your SMP as it could cause confusion (at least for PXE).