Try the red skinned potato Bec. I cook my potatoes in the steaming basket on varoma temp until well cooked then add them back to the bowl with the butterfly in place, add a knob of butter, S & P and mix for a few seconds on speed 4. If it's to thick I then add a bit of milk but too much. Sometimes, depending on the potato, it doesn't need any milk added at all.
I would use slightly more white potato to sweet potato and with cauli/potato mash, equal quantities then mash with a knob of butter and add some grated parmesan cheese.