Title: CAPTAIN AMERICA#350, ?, 394 by Mark Gruenwald (writer), Kieron Dwyer (350) & Ron Lim (394) (artists)
Synopsis: Red Skull, the employer of the Scourge from #347 claims that the Scourge operation was his idea. He has Scourge attack Captain America VI, who degfeats Scourge. Scourge is not captured, however. He makes other appearances in the Red Skull's employ before being killed by him
Is it a key part of the Scourge storyline? Kind of. It makes for an interesting red herring, but Scourge is basically used as a token gunman here
Is it part of the main story in these issues? Scourge is often present in Red Skull plot threads for the next little while (can't remember where all, hoiwever) but is mainly backup to the main plot.
Comments: Official Handbook of the Marvel Universe Masters confirms what one can extrapolate from the US Agent mini-series: the Scourge in the Red Skull plot thread is a renegade, and the Red Skull lied about coming up with and running the show. This makes sense since while one can see the Red Skull taking advantage of the Scourge operation to eliminate the competition, had he thought of the idea on his own, he most lilely would have targeted heroes first.