Good point. Not sure which is harder on fittings: airframe vibration on firewall or the movement of parts, even with flexible hoses, FWF.
As far as the fuel fitting on the firewall that's connected to the gascolator, if the fire makes it through the gascolator then it won't matter what whether the firewall fitting is steel or aluminum.
Again, though, I'm curious about whether the firewall fittings are prone to failure due to vibration issues. If so, I think that's the best argument for a steel fitting in this particular case. Just thinking aloud...