Lizzy the advice you have been given is good, when I started skiing it was the old style you are saying that you ski, but there have been many changes since, it would be good to take a lesson. Sl skis are not meant to go real fast. The new skis are way better and make skiing much easier, you need to learn to work your legs independently; think about this; if you were to stand with your feet together I could push you over easy( not that I would), it is fine to bring them in at one point in the turn but you will get much more stability if the were in a more natural standing spot as the base of the turn. Today we do not go as much forward to back as we used to, we do a lot more hip angulation and hip floating. When you do take off fast on todays skis you need to keep them some what on edge or the will want to wabble back and forth.
I have also been an instructor, and I was a ski patrol, and raced, today I just ski for fun.
I like a lot of people use different skis for different conditions, all mt. skis for average days, powder skis for powder days, race skis for blasting the groomers.
I have a set of Volkl Speed Wall GS skis coming this week, I am told they are good up to 70 MPH. I am a big guy and a strong skier so I get skis in the longest lengths they make, so consider where you ski, and how big and strong of a skier you are when thinking about length, the new skis do ski longer than there length would suggest.