would starting a vilified "war" with the ncr so early on be a bad idea as far as quests/storyline goes?
whens the earliest u can go to ceasars fort?
It would, actually since a lot of quests come out of the NCR/NCR affiliates. A good thing you could do is just start doing as many NCR side quests (not the main ones) as you want. Most, if not all, of those quests have an option that allows you to hurt the NCR. Pretend you're undercover or, heck, a Frumentarii if you want.
I don't know if it's really a good idea to go to Caesar's fort until you bear the mark. That starts the Legion main quest anyway.