Well, you use a lot of leg in dressage period. There is a lot of lateral movement that requires the use of leg, and it really starts up around 2nd level and you see it just a hair in 1st with leg yields.
That being said, ideally, you should not have flapping, wiggling legs. Ever. You should strive to keep your leg cues as subtle as possible. But, that also being said, at times, you may have to use your leg in various spots at "one time".
As for alternating leg to get forward motion, it has worked for me, but I prefer to actually put my leg back a little too far and give a squeeze with both legs, because the horse usually doesn't expect that and it gives them a surprise.
Next to your seat, you after all, use a lot of leg. Twice as much leg as hand I have always been told. But I don't know, maybe it is just a dressage thing. Admittedly, I have never studied too heavily in anything BUT dressage so a lot of things seem like just "the way things are" to me that doesn't really exist in other disciplines. :)
Cinnamon Whiskey 11 y/o 15hh Chestnut AQHA mare, 2'6 Jumpers