IMO, keeping them in a group might be a mistake. I could be wrong, but I have owned one now for quite a while and he is VERY aggressive. He was purchased with 2 other elephant nose fish, and killed them both. A larger group in a larger tank might work better, but I would assume that eventually the dominate one would stress and slowly knock off the weaker of the bunch.. and just like that, 1 by 1, the weakest of the bunch will be removed.
Would be cool to see someone try it! A LFS near me who purchases fish through an LAX transhipper brought in about 30 of them. They stuffed them in a 29g tank, and in one night, all but 1 perished.. Might just be coincidence, but the one that was left was the same 'large dominant' one I had seen the day before.. This LFS crashes tanks like this all the time though, so it might not be anything..