BO-
I see why your asking, however the lowest i'd go w/ SW fish is 1.009-1.010, 1.009 is sufficent to elimate ich. HOwever my Q? pertains to where did you read about ich being a major problem in the fry tanks? Often you'll read about bacteria, mold, or fungus and unfortunately its not effected by the lower salinities.
If you do atempt to setup these breeding systems, i would recommend a speerate system fro your broodstick, and your grow out, that way they are seperated, and not effecte dby one-another.
Lastly, it is usually a good thing to consider sterilization equipment on the water, such as O3 reactors or UV. They help w/ your husbandry, and your water issues