tanking, dpsing, and healing. normally, you would just want to fill the dpsing roles in here, at most 2. Individual movements do get hard for later bosses in naxx. Personally, I have experience with running resto as main, and 4 elemental dps in naxx. It worked ok as early bosses in naxx were not really THAT diffcult. I run all 5 clients on 1 PC as that makes individual movements easier (watch debuff, and mouse click that window then move). The elemental slaves' dps were pathetic though. I am getting 1500 - 1800 dps with cast sequence. Can't take down pathwerk with this.

With that said, the trinity setup is certainly doable. You can fill the tank, dps, heal roles. As with all, it takes practice. If your group are willing to spend time and money for you to learn the encounter. I am sure you can master it, given that you have to be geared for the instance. In instance, BOXING provides no advantages over indvidual players, only disadvantages. It makes sense to require over geared boxers to do the instance. Otherwise it would be too easy for non-boxers. Keep in mind that for some later encounters, I have not found a way to box it effectively. It doesn't mean your team can't beat it. It is just that I often had most of my slaves dead that they are not really contributing to the fight. In naxx, that would mean , Heigan the Unclean and Thaddius

1 thing I do suggest is to do whatever it takes to gear up your shamans. That means PUG it if you can get into a 25 man raid. There are many many 25 PUG for naxx on my server. I've taken all my shammies to 25 man naxx. Sometimes boxing two resto at the same time. The resto crit & mp5 epic that you get is way better than any blue or heroic elemental gears that you can get, even for elemental dps. and healing in 25 man is really ezmode as I just spam chain heal, riptide, and sometimes healing wave.