Hi there. I'm on my second playthrough (now with DLCs installed, yay!) and I decided to do everything different this time, from making a different build (Sneak, Melee Weapons/Explosives) and picking different companions, to aligning with a different faction, in this case, the Legion, as I've gone independent/NCR in my first playthrough (reloaded and got both endings).

But it looks so far that it won't be just that easy. I've noticed the amount of locations under NCR control is MUCH higher than Legion ones, apart from a slight companion problem; up to now, I've recruited Boone, Arcade and Cass, the hottest cowgirl in the entire Mojave and my favourite by far.

My question simply is: are there any "benefits" of siding with the Legion (minor factions apart), consequently opposing the NCR? Are there any specific safe places in which I may not get shot in the face everytime I go? And are there any human companions that do not oppose the Legion?

Thanks in advance, tips/answers/opinions are very welcome

You get free legion currency, a safehouse, infinite stealth boys, some of the most fun quests in the game, more combat/assassin/theif oriented gameplay.
As to companions, Boone and Arcade are the only ones that strictly go against Legion.
Cass and Veronica can be okay with Legion as long as Veronica don't know you've killed off her chapter and you don't get evil karma with Cass.
Raul, ED-E, Rex and Lily don't have any argument against Legion.

At least by her dialogue, Cass (which is the companion I intend to keep) is pretty distrustful of the Legion, even though they protect caravans. Thought she would be somehow like Boone, only not attacking Legion at sight.

Forgot Lily was a non-human

How do I rack up Legion reputation/fame? May I just slaughter NCR right from the start, or it has any effect only after I meet Caesar?

I wouldnt slaughter ncr around Cass until ya do her companian quest a certain way, even then I think shed pull a Boone and start atking.
If im not mistaken if u have high enough speech u can do it around arcade being a legion playthru.

My advice atick with ede and raul

The easiest way to rack up legion fame is to follow the main quest and eventually ul meet caesar and can start doing work for him and other legion commanders. This will rack up the rep pretty quick. Also theres a legionnaire who collects NCR Dog Tags (Though im going to be really mean and not tell you where he is)

Aditionally the NCR doesnt have more of the map, it simply controls the areas of most interest, namely the strip and parts of the surrounding areas. The legion crushed nipton, own Nelson, cottonwood cove and surrounding territory and in some circustances can take control of terriotory. For example legion can own Helios 1 if you wipe out the NCR with archimedis.