Well, for one, you dont need 4 tanks, so you should probably decide which pallies are gonna dual spec ret or holy. I've found that everything goes smoother if each boxer only does 1 or 2 roles at a time, not 3 (tanking healing or dpsing) I find that it's pretty easy to tank and dps, or heal and DPS. Some fights need only one tank, some need 3, and some need all 4, so its good that you already have that comp. (4 horsemen come to mind, you could easily put one boxer per horseman.)

What I would suggest, is try and see if you can manage throwing in more than just shaman, because otherwise, you will have 90% of the raid all fighting for the same gear, and a lot of it getting sharded. I'd keep at least one priest, and one mage, for the buffs, and definately keep a couple locks, destro locks do pretty mad dps (check furs macro thread for a good lock /click macro)

The biggest hurdles you will come across in Naxx 25 will be Grobulus, and Thaddius, Grobulus is a bitch to micromanage (maybe try the "strafe everyone at once and ignore the plague injection method" and Thaddius will take time to get everyone into position, and spread out, so you can ignore the charges, and only will be possible if each DPS pumps out 3.5k + dps on the entire fight or you hit the enrage timer. Depending on your gear, and what kind of DPS you can do, other bosses may get difficult as well. If you are pretty geared out in heroic badge gear, a lot of the gimmicks can be avoided and just healed through, like second boss in spider wing, if you can heal through her enrages without MC'ing anyone to apply the debuff, it's a tank n spank.

I just remembered, you probably can't do Instructor Razuvious with only one priest, unless you get creative with off tanks and priest re applying MC every time boneshield runs out.... If someone can level up a second priest, then it should be doable. He IS skippable if you can run past him, and use a soulstoned lock, and 2 shaman to ankh, rez, and summon the rest of the raid once you are safely past him.

I think Fur has a thread about our guilds progress somewhere. Lately we've started back up in 10 man ulduar, with 2-3 of us boxing anywhere from 2-5 toons each depending on who's available. We've only gotten through Flame Leviathan, Razorscale, and XT, and haven't been able to down anything else. Obsidian Sanctum is actually pretty easy now for us, we recently did it with 15 toons in the 25 man version, and it was pretty easy, so with 20, you should be able to do it easily. Plus, you will get all the 20 man achievements And if you are up for it, you have 5 open slots for puggers. We have a bunch of single boxers in our guild and having them helps a lot on some encounters (we use them as target masters, using /focus based targeting, for burning down adds, etc.

Whatever you do, take screenshots, or even better, Fraps it. I'd love to see other boxers attempt 25 mans.