Darn, I didn't get a chance to look at the guns because I died so quick but well that account I'm sure describes all we need to know about them, the question is, are there any ways to minimize friendly losses while dealing with these monsters?
The early ones you can counter by getting close and shooting their melee weapons while they're after a friendly. They'll switch to their 9mms or 10mms which aren't too effective. But once they start carrying 12.7mms, there's very little you can do, as it's very hard to get decent VATS accuracy on pistols. In general, once you advance a bit into the game, you can expect a single Legionary to be able to outmatch 5 or so NCR Troopers.
@Yanks4champs - it's not a quest. If your reputation with Caesar's Legion is very low, they dispatch Legionary Assassins after you. This is much like the NCR Rangers that are sent to kill you if you're hated by them, except the Rangers are pansies in comparison.
By the way, with decent NCR fame, the Rangers give you a radio that lets you call them in for help. This could work against Assassins (I've never used mine).