Re: Fire Pump alternate source
If you look at 695.3, it says "electric motor driven fire pumps shall have a reliable source of power."
I think only your AHJ can determine what is considered reliable. (I don't know if you are talking about a small building, or a high rise. I would think the AHJ may have a stricter definition of reliable for a high rise). But I think it is possible that just the utility service may be considered "reliable" (especially for a small building). In that case, you may not even need an emergency generator.
On the other hand, if there is an existing generator, I would think that would make the supply to the generator even more reliable. And if the existing generator is natural gas, so be it.