Is it OK to feed Aureomycin crumbles to newly stocked fish to decrease mortality? Would it help or hurt?

I would not do so unless I thought there was a problem. Save the medicine until needed. Don't buy uncertain fish. A salt bath may be worth the effort if you expect external problems. If internal problems are expected don't buy from that source. Most stocking morts are physical stress (O2, temp related , crowding etc.) not internal infections.