Ahhh, didn't know you were building inside a building. Since that is the case, you aren't limited by much. 2x4 would be fine with a double top plate. 10'ceiling no problem other than finding straight wood! 12' go 2x6. As far as a ceiling with a 12' span I would use 2x8. if you want it bomb proof 2x10. There are multiple products to anchor the structure into the concrete. Just make sure to to prime the bottom plate put some 50# felt between the wood and concrete for any moisture issues. Also, put two CONTINUOUS beads of caulk between the felt and wood as well as the felt to concrete to seal for any air penetration. Trim the felt and caulk at the interior and exterior of the bottom plate on top of that. Use Sashco Big Stretch. Absolutely NO air leakage with that!!! DO NOT buy wood from Blowes or Home Depot!!!
I don't know enough about using CO2 in sealed rooms. You had mentioned mini splits. So, it would seem that the intake from that would be enough for plants. Humans a different story in a foamed structure...
Do your homework on your layout and then do it again. Then one more time after that. Electrical, I would run conduit on the inside as the spray foam can/will inhibit access to anything in the future. Make sure you know where you want your panel, outlets, and general lighting as well as grow lights before the foam! Maybe hit Cave Dirt up about that...
Moveable partitions would be something to think about as well if you are going to do autos and photos.
Permits, that's why I put in the "however" Don't wanna give someone bad advise that could effect them down the road if they don't know better. You on the other hand got that part
Let me know if you have any other questions. Glad to help where I can. If you want some pictures of my work, pm me.