Any salt will work, even the iodized kinds. There is not enough iodine in any salt safe for human consumption that will cause problems. In order to cause iodine toxicity, you would have to be pickling them. Iodine is actually an additive in most foods--it's a required mineral for pretty much ALL life forms.
As an aside, brine shrimp can be hatched in pure freshwater--there is a bit of a difference in survival rates, but nothing significant.