The plural of Carnifex is 'Carnifexes'. The contents page of the Tyranid codex plainly states this, so I don't know why I was wondering whether it was 'Carnifexen' or 'Carnifices'. It just goes to show what an overactive imagination gets you. In any event, I am now running a couple of them in my list and actively enjoying them. (I have played all of 3 games with them so far, so this makes me an expert).