1. General interest publications aren't a big priority right now, because we're looking for subject matter experts. That said, the program does include publications specifically catered to new writers -- which IS a niche.
2. In the last 30 days, fiction stories have been Boosted 39 times -- the same number of Boosts as the topics science and parenting have received. (Incidentally, in the past 30 days, more fiction has been Boosted than stories about marketing or math.) These numbers fluctuate constantly, but it's safe to say that fiction is consistently Boosted as frequently as non-fiction. So yes: it's a complete myth that it’s harder for fiction to get Boosted.