I could tell you what happened in a C-141, Lockheed's predecessor to the C-5 - but happily nothing had "smashed open" and there was no fire. Are these excitements - a missile strike or mid-air collision - to be an essential part of your scenario? Any idea of the altitude at which you want all of this to happen - that makes a considerable difference to the number of seconds of useful consciousness if you don't get on to oxygen PDQ? What sort of load do you envisage - purely cargo, at a guess? Crews do get training in decompression chambers to have an idea of what may happen for real and, in the case I recall, the chamber runs proved very accurate indeed. And it can all be much less dramatic than I suspect you want it to be.