for skier cross, I'd stick with either the SX:11 (green with Titanium Channels) or SX:9 (blue with Hyper Carbon Channels) and honestly, I don't think you'de need more than a 180 (not even sure if they are making bigger than that)
i do compete in some skiercross, you dont want the ski to be too long, because it isnt only a speed race, if its too long, you will loose seconds turning them,and while you will be in the air. I am 6Ft -+180 and i get either some 180 or sometimes 185 if it is a pretty straihgt course(salomon crossmax 10 racestock)..
as for SX11 or the other one(which is a GS one, its all up to you, a skiercross ski is very often better, since its made for that, a lot of people bend their race skis in skiercross, but they are sponsored..so if you pay your ski, think about this, race skis arent made to jump 5 roller and land 60ft further, neither to jump 20-40 little table top at 60 KM/H.
hope that can help you, and if you are still looking for some ski, message me, i have some good ones.. Drink 1 BASE a day, Keep the doctor away.
Get them the same as you'd get a GS ski. Go to a good shop and ask for advice. And if you're serious about it, go with all-out GS skis - that's what all pros ride. 'Skiercross' skis are just there for marketing.