Hopefully they won't get it. They aren't the ones that need it, however much they might want it. Rapidash and Flareon actually need the boost, and make a lot more sense flavor-wise than Magmortar and Heatran.
As far as comparisons between Hihidharma's Fire Punch and Pokabu's Nitro Charge go, keep in mind that Hihidharma probably has a much higher base Attack stat.
Flareon will still be meh without a good physical attack to pair with Nitro Charge (assuming it's a Special move).
Sure it has usable SpA but there are faster Pokemon with more.
IMO, Rapidash would rather a way to boost its attack since it is quite fast as it is.
Magmortar, OTOH would absolutely destroy with a speed boost and the freedom to switch to TBolt, Focus Blast/Cross Chop/EQ as it pleased. :evil: