so i know you can get mixed relationships with factions. im going to be pro legion on this character im making and i was wondering if there was a way i can get a mixed relationship with the NCR so that no matter how many ncr soldiers i kill, itll always stay mixed, meaning that the ncr wont even attack me without being provoked. is it possible to do this? and if it is, whats the minimum "favored" status i should get them to before i can start killing ncr? accepted, liked, or idolized?

Pretty sure it's impossible to go with the Legion and not get vilified by the NCR.
The only thing i can think of would be to do all the side quests favoring the NCR before you go onto the strip.
Then serving the Legion after.

Its entirely possible to get a mixed rep, I've done it a few times though I would recommend getting to Liked by the NCR before trying it. http://fallout.wikia.com/wiki/Fallout:_New_Vegas_reputations#Reputation graph shows the different rep you can get.

Now that I got the helpful part out of the way I'll give my advice. If you are going to side with the Legion I would suggest going for them all the way, don't do any quests in favour of the NCR and be true to the Legion ideals: no chems and disdain for those who deal in them, melee when possible, attack the NCR on sight and follow the teachings of Caesar(you should start pronouncing it the latin way). Also remember that the Legion are not evil, they will bring stability to the wasteland when it desperately needs it, stand tall Legionnaire!

well im planning on doing all of the ncrs sidequests then. after you talk to house/kill benny and have ambassator crocker and the legion representetive talk to you, dont you lose all negative values towards the ncr and legion? that way i can get liked by the ncr, maybe even idolized. lol havent had that happen without siding with them, but anyways, i would get liked, then kill benny, then i would start doing the legions questline?

i am right about the faction reputation being reset to neutral ONLY if the reputation is negative? if its positive it will not go down will it?