Posing no threat to Warren Beatty, Richard Grieco plays a young Bugsy Siegel in the teen-oriented Mobsters. Christian Slater costars as Lucky Luciano, Patrick Dempsey as Meyer Lansky — and all seem a tad callow to play such deadly studs. Yet the film gets added stature from F. Murray Abraham and Anthony Quinn, and an extra edge from some memorable mob hits. While only skin-deep, it’s thoroughly diverting. B