Is it possible to disable one or the other for just one list, or temporarily (I am studying for a test right now which will just be in simplified, but I generally want to study traditional too--I'm moving to Taiwan next year).

Sure. For just one list, change your setting to Simplified under Account: Study settings, and then add from that list. But if you've already got it added, then just switch to Simplified, and Skritter will only test you on simplified. If you don't add during this time, then when you switch back to Both as you prefer, you'll catch up on the trads that you didn't study, and keep adding both. This is what I do, except I emphasize trads.