When St. John tells Jo to "Watch your tail," and Jo replies, "St. John, I'll be fine," it's the same line used in Escape, except in reverse.
The fact that Colonel Curtis was St. John's former commanding officer creates a confusing web between St. John, Stringfellow and their commanding officer(s). If Stringfellow and St. John served TOGETHER in Vietnam when St. John went MIA, that should mean they had the same commander (in the episode HX1, they were on the same mission). Colonel Vidor from And They Are Us was stated to be String's commanding officer, and in HX1, String states that he, his brother St. John, Vidor, and Mace Taggert were the only ones who knew how to execute auto-rotation on a chopper, implying that they often worked together as a unit with Vidor being in charge. So it seems that this Colonel Curtis character came out of nowhere just to have a body thrown into the story.