I'm playing in a politics-heavy Rokugan campagin right now so here goes.
Nezumi: you are immune to taint which is a very good thing as it can mess you up bad. Also you can aquire the scent feat which is very good, and have a d4 claw attack.
If you are playing a cleric might I strongly suggest you take a look at the Shaman class instead? It is a full divine spellcaster who takes light armor and a d6 hit die in exchange for an animal companion, improved unarmed strike, a _very_ cool ability called "spirit sight" (see ehtereal 24/7) and some bonus feats. It even keeps turning, spontaneous casting of heal spells, and 3/4 BAB just like the core cleric. The flavor is that each of your domains (2 at first and a third at 11th) represent a patron spirit who you have a special bond with, so if you choose the earth domain you have a bond with earth "spirits". A very cool class and very OA in flavor. There are also several new domains which you defiantly want to take a look at. The Nature domain sounds perfect for your character (turn/rebuke both animals _and_ plants, and the spells are geared around stuff like commune with nature spirits). Some other domains to consider for a nature-oriented character are River, Stone, and Wood. Or, if you really want to play up the spirit-talker thing you could look at Ancestor and Celestial domains; talk with every spirit under the sun!
Moving on to special rules: You aquire taint by casting blood spells (bad!), being the target of a few nasty effects, or wandering around in the Shadowlands unprotected. Humans can be protected from Shadowlands taint by carrying jade (a sacred material in OA) which soaks taint up for a while before the jade turns brittle and is destroyed, then you make increaing Fort saves to hold off the taint. If your taint score gets too high you turn to the dark side. If you die in the shadowlands you rise as a zombie (unless your friends are there to burn you body). Speaking of dying: under "core OA" resurection goes against the celestial order and is considered an abomination, don't depend on Raise Dead. Speaking of special materials: jade is good as mentined above, obsidian is bad (it makes you go nuts under the full moon and kill your friends), crystal is usefull for combating the Lying Darkenss but as others have mentioned the Darkness has been defeated so that shouldn't be an issue.
Ijatsu Focus: the most abusable, twinkable skill I have ever seen. But no need to worry about it unless you are samurai.
Martial arts styles: really only for monks and unarmed fighters but if you go Shaman you might want to give them a look. Basically if you take a certain set of feats you get a bonus special ability at no cost. (Fear my monk with Freeze the Lifeblood and Meditation of War Mastery

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Feats: mostly come in two flavors. Ancestor feats can only be taken by members of a particular clan (so human only!) and provide some flavorful, but generally low-powered benefits. Most of the general combat feats are for samurai or martial artists. Defensive Throw and Defensive Strike are good for the image of the frail guy who you can't touch but he keeps knocking you on your bum. Choke Hold takes Grappling to a whole new level. BTW: the Shaman can get these as bonus feats (I keep bringing that class up, don't I

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Prc: nothing springs to mind to fit with your character concept.
Alignment: in default OA there is no alignment. The taint replaces evil as far a detect/smite/protection type stuff goes. Other than that it is all about Honor. A samurai can do some prety wired stuff and still be considered honorable (hit a pesant) and yet cannot do some other things that a standard adventurer would do without a second thought (loot a corpse? That requires touching the body! A noble would never touch dead flesh, only the eta (a sort of "unwashed" caste) do that).
I think that is just about it for special rules. The thing you really have to worry about in an OA game is the politics. Honor, bushido, law, family, clan, empire, politics: these can get very out of control very fast. I have never played in any other game where my diplomacy and sense motive scores have gotten more of a work out. You are a nezumi which means that you will be scorned by most and welcomed only by a few (mostly the Crab clan whose job it is to guard agains attacks from the Shadowlands, you have been their allies for centuries). Your quasi-outsider status may get you places other party members cant go, it may also get you killed. Of course this depends on how the DM runs the campagin but the politics in OA are so thick that some is bound to seep in. If you want there is a very good article on AEG's website that recaps the last century or so of Rokugan's history. I have no idea how much of this your DM plans on using but you can find it here:
http://www.legendofthefiverings.com/rpg/feature_history_01.html
Hope that this very long post helps you out some. If you have any other questions feel free to go ahead and ask. And good luck!