I've used a toaster oven to cook sculpey - not on a pony - (because the fumes are gross) and haven't had a problem.
How long you are baking the clay for? I've, once or twice, forgotten about the clay in the oven and had to dash into my kitchen like a crazy lady to pull clay out, but even after cooking for 3 or 4 times the instructed time, I have NEVER burnt, melted or charred clay.
Also, is the oven in Celsius or Fahrenheit? 275F is very, very different temp wise to 275C - maybe your oven is celsius? Sculpey instructions are to bake at 275F - which is 130C. I imagine if you bake it at 275C (which is very VERY hot) it would char, melt, burn and turn to clay puddles.