He may have been David Letterman's No. 1 pick, but it seems Tom Snyder didn't necessarily top CBS' list to helm the 12:30 a.m. slot following Late Show With David Letterman. Though Broadcast Group president Howard Stringer was publicly upbeat about Snyder, he says he actively considered a number of other candidates, including several women who might be able to break into the all-male late-night fraternity. One female approached by the network: acerbic actress-novelist-screenwriter Carrie Fisher, who politely declined. She felt ''her career had a different focus,'' says Stringer. Adds a spokeswoman for the actress, ''I can't imagine her doing that [show].'' Meanwhile, the Snyder deal is not set in stone, but, says his lawyer, Ed Hookstratten, ''[If] it's Tom's show, he'll do what he does best.''