If I want to use fresh sausage, wouldn't it be hard to remove the casings and slice it up? Can I roast the sausages whole and then cut up after?

I would suggest roasting the sausages whole....I personally prefer sausage in its casing, texturally. I also think its a more beautiful presentation.

It varies some from sausage to sausage, but generally it's quite easy to remove- just slit the casing lengthwise and turn it out. It's already ground, you shouldn't need to cut it up, just break it apart. Cooked sausage holds together more, which is why it's handled differently in the recipe.