BTW, I didn't check what the D. Rhoad's "Suboc" book said about nematodes (esp. since you did anyway) but I suspect he's collected "his share" of W/C subocs.
And in the wild, they eat lizards!
I suspect some of his snakes tested negative but still managed to re-infect themselves after some time elapsed. Tests aren't perfect. And no one manages to clean up all fecal material immediately, no matter how complete an effort is made. Honestly, I don't do "regular deworming", but I don't think it's necessarily a bad idea- it's up to you. Clearly your new snake shows good reason to be treated. Panacur is easy to come by- all feed-stores carry it, even if pet stores don't. It's not even expensive.
Yes, I'd let him settle in first- it's not an emergency. I would stick with feeding mice- otherwise you'd be feeding "mouse-sized" rat pups that WOULD be too fatty, & they'd also be too low on minerals (due to having smaller, less-developed bones). You always want to feed the "most mature" rodents of the appropriate size to snakes. And remember that ALL our domestic rodents have more "fat" than wild rodents, because they're raised on a more grain-based diet with abundant food.