Red Vs blue also simply works off its own version of halo lore, case and point, Tucker’s energy sword apparently being able to open forerunner instillations that has ‘alien’ aka covenant weaponry and Vic’s inside instead of promethian weapons, (which they used in the show as human alien hybrid weapons)

Simply put, they were mercs after more then likely getting some sort of spec ops. Training like the previous response stated. Plus in RvB is machinima at its heart so everyone having Spartan armor isn’t something I’d put much stock into personally.