Considering that your guys mingle, I'd be worming the kitties, too.
My one experience with roundworms was with Sunnie's puppies (since advice I got from both my vet and the rescue's vet was to not deworm preemptively and so I waited till I realized that hadn't been good advice). That was disgusting. But my cats weren't allowed in the area where the x-pen was...didn't think Sunnie would take kindly to the cats in close proximity without us around...and so I was confident that things were contained. The puppers were not allowed outside of the x-pen at that time. (And when I took them outside, they stayed in one area...in a fenced-in spot....that I cleaned up totally. Kitties didn't go outside anyhow.)