I figured I'd share this response from Pro-Line's support:
The Dirt Paw tires have a thicker carcass and were designed to be used without foam, you can however add foam if you would like."
I think I'm still going to add the soft foam to it. I'm mainly concerned with the rear slipping in turns and so I think I want to go with a stiffer tire. It's not a big difference either as for how stiff it feels.