Don't sweat it too much, 6cm is a really small difference. A longer ski will be more stable typically, it just won't swing around too fast if you're doing a lot of park but there's not much of a difference.
I've heard nothing but good things about the Alpha, of the people I know who have it, they love it. It might be weird to get used to at first if you've been skiing camber skis because the Alpha is more of a rocker but once you get used to it, rockers are typically more playful. I stick to camber skis for personal preference, they just ski better for me and yes, it's easy to surface swap on camber skis with the right technique.