A "standing rib" is a roast with the rib bones in, not removed. A 6lb roast is very small size for 8 people, certainly too small for a festive occasion. Allowing for waste (including bones) you're looking at an average serving size of less than 8 oz per person. That's not large enough for anything other than "New Years at the Spa," or a group of women in training for their wedding dresses.
As a rule of thumb, allow 1 bone for each two people. Thus a roast for eight is a 4 bone roast. Taken from the small end, a 4 bone roast would run slightly less than 8 lbs, and from the large end would run slightly more than 9.
The normal serving model is to carve and serve at table; a 1/2 bone's worth of meat per person, with a slightly larger pieces of meat including the bone to each gentlman. It may seem sexist, but since standing ribs are often served at festive occasions it's just being senstivie to the realities of manicures and lipstick. If the lady desires a rib bone, at the time of service she is encouraged to stab the gentlemen beside her in his thigh with her dessert fork, alerting him to her wishes.
If your roast is already bone-out, it's not a "standing rib" at all but a simple rib roast. In that case, 6lbs is adequate if not particularly generous.
There are a number of ways of cooking a standing rib -- two of which involve swapping high and low temperatures. You can also cook a roast at a steady high, medium or low temperature. Other than cooking all the way through at high, they yield substantially similar results -- differing mostly only in timing.
Considering the size of the roast, even if it's "only" six pounds, the outside will brown adequately even at a fairly low temp.
A roast cooked all the way through at a high temperature will give you the greatest differential between outside and inside. It's the best way to get a "black and blue" effect, but otherwise probably isn't useful unless you're in a huge hurry.
Your life would be probably be easiest if you cooked at a steady, medium temperature with a thermometer planted in the roast for the entire cooking period.
Whatever your personal preferences and the preferences of those of us reading you here, you seem determined to cook your roast to the degree that will most please your guests. Congratulations! You're teaching Good Hostess 101 until further notice. :smiles:
140F in the center of the roast (aka "medium") was an excellent suggestion, as was reserving the "end pieces" for those guests who prefer their meat well done, and as was the alternative of reheating a couple of pieces for the notorious duo.
Large roasts need plenty of rest after cooking and before carving. A prolonged rest helps satisfy a broader range of desired doneness as well -- overcooked meats tend to be moister, while underdone ones tend to show not quite as pink. You can rest your meat for several hours by wrapping it well in foil and placing it in a well-covered, well insualted "cooler."
Considering what you're up against with your guests' choices and all the sides that comprise a rib-roast dinner, I strongly recommend the long rest. It will not only make your dinner better, but will make your life substantially easier. Imagine the inner peace of a roast, perfectly cooked, out of the oven, and doing it what it should be doing to live up to its beefy potential -- before anyone's shaken the first pitcher of martinis.
Hope this helps,