Every restaurant and menu is different, but from my experience, any restaurant that has a flavored mashed potato always has a small amount of plain mash to accomodate such requests. Don't worry, you'll be fine.
We had mashed potatoes at three places in December:
Whispering Canyon -- Yukon Gold and very buttery. These were the best of the three.
'50s Prime Time -- pretty standard and good.
Ragland Road -- a little "stiff" for a lack of a better word. I'd say they could have used a bit more butter or cream. These had onions pressed into the top, but I would think they could be left off.