The mysis in my tank will eat: Flake, dead hermit, other mysis... anything really. They mysis I have in a bucket by themselves will eat other mysis(not suggested as it fowls the water quickly), and hatched brine.
I would really suggest the freshly hatched brine. Its not that hard. Gallon jug, airstone, 1.015 salinity(what I keep my mysis at as well), and then I seperate these out into two mason jars... Feed a little bakers yeast and they are good.