In regards to their proposed merger to create Pennsylvania's largest insurer, executives at Independence Blue Cross and Highmark Inc. insist they won't play hardball when it comes to negotiating with doctors and hospitals over payments for their services, despite their market clout. But the head of the Pennsylvania Medical Society, who testified at Insurance Department hearings on the merger, is skeptical. Society president Peter Lund thinks the two companies will have too much negotiating power statewide if they merge.