So today on the way to Novac, I came across a battle between a Legion Veteran, a legion recruit, and Recruit Decanus fighting some NCR troops. I killed the Legionnaires after the NCR were cut down (literally), but I have this thought in the back of my mind that Decanus is important somehow. Is he? if not, why the hell is he named and has his own helmet?

I think that Decanus is just a rank, rather than a name, so you should be fine, it's just a generic npc type.

Decanus is just a low ranking officer title, not a named NPC. Nothing to worry about having killed, you will come across plenty of them down the line.

ah ok. thanks then