If you're Idolized with the NCR and lose some NCR fame, you're still fine with Boone. I killed everyone at Helios One once, while Idolized, and he never said a word, so I think you're fine. He only starts complaining if you drop below the equivalent of being "neutral" or some such - you'd have to lose a lot/kill quite a few NCR soldiers at this point.
Hahaha helios was funny, I trigger achemides and the ncr was hostil, boon being boone, just started taunting the enemy, and shooting them, que slow mo head explosion, then boone was laughing. Then 2 minutes later looting the corpses, he warned me we wont fight ncr and how he wont betray them... yeah.
WaitedI can convice him to join me even when liked by the legion ? Win. I will use the legion to get stealthboys then kill the veteran in the safehouse, the go to the fort with boone for caeser. I did want to plant c4 on caesers chair, but having boone would stop that, shame I need to do this for points to unlock his quest.