Not to be picky about it, but the WP F-Unit he bought IS actually a passenger engine. Note that the Original Poster is not talking about running a passenger train, but hauling a freight train behind an F-3 that is painted in this Western Pacific passenger scheme. (Your technical point about steam generators in B-units is correct, but doesn’t really apply to the Original Poster’s needs).
As noted in his other thread, the WP did actually haul freight behind its F-3s, so the Original Poster is fine.