Starcraft – In which Wings Of Liberty missions can you play as the Zerg

Yes. Any achievement, any achievement, that has a difficulty requirement as part of the achievement can also be completed on a higher difficulty. This means that (if you're good enough) you can get all of the Wings of Liberty Mission achievements on a single playthrough.

E.g., for the Red Lobster achievement, you can taunt the Brutalisk into the lava on Normal, Hard, or Brutal -- you don't "have" to be on normal (despite the listing to the contrary)

In the Media Blitz mission, in the lower right area of the map (by the mineral fields), you will see a semi-hidden path. Go to the end and destroy the Science Facility building at the end of this path. You should pick up some secret documents that will give you the secret mission. This mission must be played while you're still on the Hyperion.