By "cream" do you mean "milk?" If you are lactose intolerant, I suggest some margarine and either sour cream or yogurt. If you have a lactose allergy, still margarine, more alternate dairy products, and some kind of spice mix, like an Italian seasoning.
Yes, you can.
The problem is that the potatoes start absorbing water, and you end up with a really runny mash that's difficult to flavor (as most people flavor their mash by the addition of flavorful liquids).
update : There's a few things that you can do to hold them. The first is to drain the water, and leave the potato bits (unmashed) in the pot. If you leave them uncovered, they'll cool off, but it'll also let moisture evaporate, which can help to fix some of the problems I mentioned earlier.
You don't want to let them cool off too far, or they'll not mash up smoothly; if you're going to need to hold them for more than 5-10 minutes, put a lid on them. Once it comes time to mash, refresh them with whatever liquid you're going to add to the mash, warmed.
If you're going to need to hold them a really long time, you have two options ... one is to mash them but intentionally reduce the liquid, and then refresh them with more hot liquid a few minutes before serving. The other is to transfer them to a baking dish after mashing, and put it in a low oven. (if you rough up the surface, and put it under the broiler right before serving, you can get a crust on it for a bit of variety).
Best Answer
Both kinds of potatoes you mention should work as a replacement to a portion of the flour required by the recipe you'd like to follow. I think replacing ~30% of the mass of flour with that mass of your choice of potato is a great start.
The sweet potato might be a better choice because of the added sweetness you'll get. You don't want to go far above 30% if you do that because it will start to dominate the flavor of the roll.
As for softness, it does seem like the end result will be a bit softer although I can't say I've tried it before. You could even consider using some high quality ricotta cheese instead of potato. If using ricotta, I'd limit to ~20% replacement for the flour as this is less starchy than the potatoes. This will help to soften the roll as well.