2x6 short studs under the floor.
Only forget the firewall if the rooms will fill the whole garage.
If the rooms will not fill the entire space, you will have to have a firewall.
2x4 studs will be fine to frame the wall above the room up to the garage ceiling. BUT, the wall would be stronger and quicker, simpler for me to build if the studs ran from the garage floor to the garage ceiling.
PT wood is only necessary at the bottom nail plate.
The advantage of connecting to the ceiling is strength and integrity of construction and a continuous firewall. Strength would come from unitizing the wall from floor to ceiling. The wall would then add strength to the house and the house would add strength to the wall. (It's a Zen,, or Ying and yang thang, mang.)