I have no idea what Halo Wars always telegraph where the AI will attack you and what they'll use but I love it.
Oh you made a lot of banshees? Time to attach missiles throwers to my turrets, elle-o-elle!

Oh yeah, turrets in Halo Wars are attached to your base and are upgraded through techs, but they also can get specialized expansions, like a flaming mortar to deal with infantry, an "electric cannon" to deal with vahicles, and a missile launcher to deal with flying.
The add determines the preferred target, but is added to the normal machinegun.