Has anyone done a one page setting and/or rules introduction for Anima to hand out to players?

yeah, I have. pretty easy too. I'd recommend making one for your new players (if any) just because it's a handy reference for " what kinda world am I playing in?".

I made some changes in atmosphere for my game that required some "notification" if you will so it was a necessity.

Are you willing to share your efforts?

As both me and my players were new to the system, I had the first session be something of a teaser/training session. I made some basic shadow monsters... level 0 sneaky guys, with some ki so they can get 2 actions, or make 2 attacks in one round. Its good practice for you, if you go through and design the level 0 monster yourself using the rules in the back of the core book. My players took the chance to try out all their different attacks and abilities... and stuff like taking multiple actions in one turn. Just wing it, and make sure all players have an opportunity to try their skills out. For the first session, you won't have any idea how much damage your players are going to be dishing out, so you might want to fudge the monster's HP a bit, to what feels appropriate. Monsters will work better for this purpose than guards or other humanfodder, because they'll actually survive long enough to practice your skills on. Also note that the example NPCs in the core book are complete fodder for players, even at level 1.

edit: If you really can't come up with some way to work this into your planned setting, here's the gist of what happened in mine... shadows in the Wake (read your DM section in the core book) are acting up... people in the real world (include your PC's in on this) get psychic resistance checks (commoners and other "normalers" fail this automatically, since they have no gnosis). Failed people freeze, and some of them get controlled by shadows from the wake (they look shadowy in the real world while controlled, these would be your monsters). When the players get them to 0HP, the shadow leaves its host, and the host falls bruised and unconscious, possibly rescuable. If you need an early BBEG, the lv3 Shadow monster in the core book might make an appearance (he'll dish out one or two hits and leave. Don't use his instant-kill power... just do normal cold damage, and make him more aggravated at his minion's failure than at the PC's success, so he'll make a quick exit after venting his fustration on the PC's).

Hope this gives you some ideas!