Wound penalty also applies to the soak roll?
I don't think so, no.
Even if it does (which I think comes down to interpretation of the wording of the rules), I would make it take a moment to kick in, so it wouldn't apply to the other versions of the spell being multi-cast in the same round.
Yes, as per ArM5 p. 178 "The character suffers a penalty to all actions (rolls and totals) equal to the sum of all penalties due to his wounds*". For combat rolls, the penalty applies to your defense total (which is the same thing since it gives a bigger advantage to your opponent) while for non-combat rolls, this applies to your soak roll (soak + stress die).
See also this post by Christian Rosenkjaer Andersen: Do wound penalties affect soak totals?
This is how my group handles it; Yes, wound penalties affect soak, but multicast spells don't stack each other til they finish.
Yeah if you’d hit with a swarm of Crystal darts the soak is rolled independently of each other.
Accumulated wound penalties don’t apply until the next next round, of fit attacks later in the initiative order.
I have to agree with Darkwing. Crystal Dart is fine as it requires you keep 2 arts at the same level to get your penetration up. This is powerful, but balanced as you need to spend extra exp on keeping your scores equal.
As to increasing the damage, I think you could do this to a point, but I would limit it to what seems logical for the situation. Giving it +1 size to make a crystal spear, adding a magnitude to make them metal and increasing the target to group would probably be my limit for the spell. You could maybe push it further by giving it a ignem requisite for another magnitude. That would put you at +25 or +30 with the requisite, but you then need 4 arts to make this effective. Further size or number increases would probably yield more damage, but as an ST I'd make it diminishing returns.