not sure on the powdered milk due to may contain carbs that could knock you out of ketosis, have you tried a vanilla shake? that makes a wicked coffee or the Chocolate tetra, which turns coffe into a Mocha...or why not ss+ and use the 200ml of skimmed milk xx
I don't think powdered milk is allowed on SS, but as DappyMare said, you can have 200ml of skimmed milk (and four packs a day) on SS+. As for sweeteners, I think the rule is that tablet ones are okay, but powdered ones aren't, so as long as you choose the right ones, you'll be fine in that respect.
Splenda as far as I know is the advocated sweetener and skimmed milk is a no go I'm afraid.
I use the vanilla shake (a teaspoon is all you need to "whiten".. i take a couple out of the shake I am going to use that day and put it in a cup to the side for the day). I do find it clumps though so use the blender.