I bought the ability to be assigned ice strykes from kuradel but what if i go to duradel in shilo village will he assign them too?

no

So you can only get them assigned them from duradel if you're in the posession of a fire cape, that correct?

I'm fairly sure that you can get them assigned from Duradel regardless if you have unlocked the ability to kill them without a Fire Cape.

 

I don't know why you'd use Duradel rather than Kuradal, though.

Okay well i'm still not pretty sure what it is now so i need someone who can really confirm it.

 

Thanks anyways.

Duradel can assign them, however he assigns them much less often than Kuradal.

 

EDIT: http://runescape.wikia.com/wiki/Ice_strykewyrm

Source.