Script-based retaliations:
October 1999: Gazers!, Waspites, Sectoids, Snakemen (and a chance of Chryssalids on non-Sentry ships!), Mutons, Cerebreals, Floaters, Chasers.
December 2001: Same, plus a chance of some Ethereals.
I propose making a few more tiers (like the 'add Ethereals' one) there and allowing only something like Sectoids, Floaters and Chasers to come after X-Com really early.