Yes, it will work, but I would recommend keeping these separate. Keep in mind that down the line you'll be installing DS 7.1 on the NS 7.1 box that also hosts DS 6.9 SP5. That's where it gets messy. If you were eternally using DS 6.9 SP5 and NS 7.1 w/o DS 7.1, then I wouldn't have any hesitatations. But this could make your upgrade process more complex.
You should be able to have a site server running task and package services and also functioning as a DS 6.9 PXE server. Again, if you plan to make these site servers run the boot services later for DS 7.1, I'd suggest not making these the same -- or at least be aware that imaging will be unavailable during the transition from 6.9 to 7.1.