The point I was intending to make about the alignment restriction isn't that it should be impossible (which means I overstated my case, for which I apologize) but rather that Half-Fiend is an inherently evil template, and there would need to be a really good reason to allow a half-fiend paladin. Note that "because the template is kewl and we can just handwave the alignment thing" isn't a really good reason. I, personally, would want a long, detailed background complete with info on his fiendish father or mother (half fiends are the immediate descendants of fiends, remember), so that I could incorporate that into the game. And, again, "my mother was a reformed succubus" won't cut it.