To run naxx with less than 10 people, you basically have to either overgear it, or at least be decked out in full heroic gear. The bosses hit harder, and there's a lot more going on elsewhere. There are a few fights that normally require an off tank, and an off healer, and very often an additional raid healer.

I echo the previous responses. Pug it once to learn the fights, then try it with your 6. If you have enough DPS you should be fine. Nearly every naxx pug I went with there was usually 1 or 2 clueless people, that did barely 1k dps and we still made it. If you have a team full of 3k DPS toons, you have a shot.

And if 6 boxing it doesn't cut it, make a friend or two and try 8 manning it, or get 4 friends and 10 man it. I've found that in raids, if I limit myself to 2 roles, I do much better.