I currently use a 7MC HTPC with 2 extenders and a WHS for serving my house. However, I am in the process of buliding my next home, and I am going to use a slightly different strategy in it. I will have a "Family" HTPC that my family's extenders connect to (basement, their bedrooms, office, etc) and a "guest" HTPC that my guests can connect to in their bedrooms when they stay via Xboxes. This way, they won't see, delete, or impact the recordings of the family, but they will still get all the benefits of the their own live tv, recordings, audio, videos, etc.
This will be handled similarly to the family HTPC which has an SSD for the OS, a 1 tb drive for recordings, and a Ceton InfiniTV 4. The quest HTPC will be almost identical, minus the dedicated video card.
As far as using Xboxes to connect to the HTPC, it could not be simpler. It just works. My only beef is that there is a not an efficient way to play my blu ray iso through the extenders....but I guess that is what Dune HD players are for ;-).
If you have any specific questions, please let me know.
I will try to answer the ones in your post.
I have not run into any issues with using extenders. My HTPC is full media center, it is connected to my main viewing TV / reciever that we use to watch movies and TV. There is no impact when the extenders connect to it to watch tv, other than using up tuners if they are watching live tv on a different channel than the HTPC is.
I don't know how many extenders you could connect to 7MC, but I believe I heard it was 8. I only have 4 tunes available in my current HTPC, so i wouldn't connect more than 3, unless I put another Ceton in it.