Well I thought I would do a Legion playthrough because I have not done one yet, so while going to Nipton as soon as I walk into the town the dude with the dog on his head is automatically hostile. I am not wearing NCR armor/clothing and my NCR reputation is neutral. Also my reputation with the legion is also neutral too so they no reason to hate me.

If you have Boone as a companion, every Legion man would be hostile by default, even if you said to Boone to wait somewhere else. Send Boone to the Lucky 38 and everything will be right.