Do you have to use coroplast under fleece?

Not necessarily coroplast, but you probably do want something underneath it, like a tarp or a shower curtain or something like that.

Coroplast isn't a necessity. Before I got my coroplast I used contractor bags and made sure the pigs couldn't pull up the fleece to get at the plastic. Now that I have coroplast, I like it much better because it has tall sides and the piggies can't kick out hay, poo, or wood pellets.