Yeah there's nothing wrong with asymmetrical duck stance... When my GF was learning we set her up with -6/+15 or so, to give some forward bias. Last year we slowly moved her to -9/+9 where she's happiest.
I ride at -12/+12 and that seems to work for me.
If your focus for the week will be on freestyle, get a more freestyle oriented board. Of course you can use an all-mountain in the park, hell I use freeride boards in the park! But again, if your focus is on freestyle, get a freestyle board!