I have to side with skittles on this one. The biggest concern, really, is that Fredrik is a solidly baritone role. As much as I'd love to see Raul in any musical, I don't think he's the most natural fit for the role vocally. As for casting the roles so young, the unfortunate thing is that you lose the generational contrast between Fredrik and Desiree and Henrik and Anne. That's essential to the story. I'm not sure I really buy that Anne would be freaked out by the prospect of sleeping with the much older Fredrik if he doesn't read that much older than her.
I think I'm going to produce a younger, hipper revival of Follies and cast Raul as Buddy or Ben.
Seriously though, I'm going to be optimistic that it'll work out fine. I just don't see Raul as Fredrik as perfect casting.