Couple thoughts, I don't take my team into naxx together but split em up and run pugs (have cleared both naxx10/25 on 3 of my 5):

- Naxx25 is easier than naxx10 for the most part. Sad but true. Gear coming out is also better. No big deal, just fyi.
- I would agree that wrt multiboxing, the best comp would be taking in 3 ranged DPS. Actually the best classes would be hunters or mages, perhaps boomkins. Shamans are mediocore imo, maybe next patch will up the DPS but for DPS fights like patchwerk/thaddius putting 3-4 shamans in is kind of gimping your raid. Still do-able of course.
- for starting naxx groups, the setup tends to be 2 tanks, 3 healers, 5 DPS. You can drop to 2 healers once geared up and replace with a DPS.
- replenishment IS HUGE! Ret pallies, hunters, or shadow priests are imo a must for beginning naxx groups, otherwise healers gonna be running OOM a lot (esp on patchwerks). I think frost mages also can replenish, but frost is a very gimp spec to FFB.

I don't really think you can tank and multibox DPS at the same time, as you found on on Anub its very hard (maybe for the adds you can OT them and DPS), it gets insanely harder later with stuff like Noth (tank on adds, DPS on boss), Gluth (omg), Thaddius (tank thrown diff sides), etc. I would say take 2-3 ele shamans and run with that in raids.

Naxx gets a lot easier once your entire raid is above certain spell dmg/attack power/HP/etc constraints, so getting as many people to get their t7 from badges/voa/obsid sanctum is a great start too.