You are largely correct. It was Bungie who said this before Reach launched. Anyway, yes, while the vast majority of the SPARTAN-IIIs were trained on Onyx, some personnel were deemed to valuable to be sent on the high risk missions. So you see the Headhunters formed, and highly equipped commando squads like NOBLE Team. In fact, during Reach, we hear about two more SPARTAN-III teams similar to NOBLE. [i]"Sir, is that true about GAUNTLET and ECHO teams being assigned to evac ops?"[/i]