Following what seems to be a snowballing trend, the new show at Down in the Depths, the pocket-sized nitery below the Hotel Duane, New York, features a full-length musical revue titled "Smalltacular." A cast of seven prances thru assorted songs, skits and singles, following closely, the timing of a TV spec in that the 90-minute show is broken into three segments, with the breaks between for drinks rather than commercials. . .