You have to play through Adventure mode a second time to get the Zombologist achievement (or Cryptozombologist, depending on which platform you're on). The Yeti zombie first appears in stage 4-10 (the night fog stage) on your second playthrough, and appears randomly and infrequently afterwards.
Completing each stage a second time will also give you a small amount of money instead of the seed packets you got your first time through.
The plants Crazy Dave picks for you are randomly chosen when you start the level. You can get a new selection by restarting the level. (Confirmed by Jason in the comments, thanks!)
Tip: Neither zombologist nor cryptozombologist are real words.