I’ll attempt to answer few questions by memory.....which is not very scientific!
On the 777 there were some escape routes in Himalayas that we had that initially descended Fl 270, then 240, then 180,then 140 finally, so depends on options and terrain- if your aircraft didn’t have the extra required O2 different routing required
normally freighter Ac, have class E upper deck cargo compartments....with no real fire suppression other than descent to Fl 250 and depressurization to try and starve fire of O2.........so O2 required was for pilots and supernumeraries to fly at fl250 for 2 hours? and supernumerary had same type of bottled oxygen