can I apply buttercream directly on a styrofoam cake form or do I need to apply a glaze or something to it first?
slap it on there
Just to give you more information - yes you can BUT....... it will be a bear to clean if you want to save the styro for future use.
You can soak the buttercream off. Even put it in the dishwasher to get it off. Just remember to take it out before the drying cycle.
The easiest method I've used is covering the dummy in Glad Press & Seal before decorating and then all you have to do is peel it off later!