But then, season 3 was the start of network meddling and it got much, much worse with the last two seasons. I think that was also the time Quinn became "Action-Quinn", whereas he was a science geek in the first seasons - and was much, much more likeable.
Invasion is a Season 2 episode. Greanted, its very close to the end of the season, but that is when they are introduced. Season 3 has some Kromagg run-ins, but not as heavily as Season 4.
By the start of Season 5 the only original slider left was Rembrandt 'The Cryin' Man' Brown.