Adding mashed potatoes for moister/softer cinnamon rolls Is it true that adding mashed potatoes as an ingredient makes cinnamon sweet rolls moister and/or softer? 

If so, can we simply add a healthy dollop to any yeasted cinnamon roll recipe? Is there any other trick to this?

Could yams or sweet potatoes be added instead, for more flavor, with the same benefit of being moister/softer?
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.