Does it take up a rare slot when you take an ancient steg as a mount? I'm sure this has been discussed before.
Yes it has, and the answer to your question is a wonderful no, not a single mount in the Lizardmen army takes a choice.
It is a good boost to be able to take a carnosaur for no hero slot too. In the case of stegadons, with just special and rare choices you still had the potential to take 6 of them in a 2k game. If you put characters on them that increases to 10, but there isn't the points for them, and 4-5 is about the realistic max you would take, which was possible anyway.
Would get boring fast... Gunlines with lots of cannons could possibly deal with it, little else could. You would just point and charge all the time. 2-3 in an army is pretty cool though.
Ya the stegadon army would be just as cheesy as a dwarven gun line, do it if you want a guaranteed win, don't if you want everyone to have fun.
I wouldn't say it is a for sure win. It is actually probably going to get creamed by some armies and be able to cream others because of its one dimensionality.
I once was forced by an extremely nasty opponent to take 4, 1 being an Engine, not a single Stegadon ever died and it was all easy wins even agaisnt great tactitions, but the fights were boring.
But I hear that Stegadon's need the Impact hits to be effective, so wouldn't an army full of them get creamed by something that is much faster? I imagine there is some reason not to bring that list to a tournament, where concern for cheese is thrown out the window. Though this may just be wishful thinking.
That is where the skill comes into it. You don't necessarily have to be faster to get the charge on someone, just have to use skinks and terradons to obstruct their charges and force them to make a bad move, then counter charge.
Or just soak the charge with one steggy and countercharge. Stegs are resilient and stubborn, good for tying up units.
The problem with an all steggie list is that if someone gets off some sort of spell that requires an initiative test (Pit o' Shades, yadda yadda) that is a steggie down the drain. Template weapons can also wreak havoc, since they would hit every skink crewman as well as the steggie.