that would depend on the snowflake. The one i have is very peacefull. He is only agressive when my valentini puffer tries to eat all of his krill!! Besides taht he is fine. He wont even eat silversides..... and i have tried many many times. When he was realy hungary, i tried and he would bite it, rip it off, then let it go immediatly. however, many snowflakes will eat anything that will get in their mouths. A yellowtail blue.....damsel? That would fit quite nicely in a snowflakes mouth. I actualy saw one that ate one at my LFS about 2 hours ago. A dragonette would too.
i would not recomend it unless he has previously seen the snowflake and has been able to observe the behavior.
and as you saw in a previous post, they can eat all of your hermits too. it jsut depends on the snowflake