Moisture is only good for brand new clones and seedlings, after that its best too keep the enviroment dry dry dry, because moisture promotes molds and fungi which you dont want to get started with, plus when your in bud you want it really dry because it makes the plant produce more trichromes and keeps it healthier, because plants let out moisture all on their own, thats why if you wrap a bag arounda branch it gathers moisture in that bag from the branch letting moisture out, so if your plant is rooted then think dry, a cost efficent way of drying the air is getting a few of those dry air crystal containers and putting some on the floor and some higher up, they are much cheaper then a dehumidifier so id go with that , thats what im doin ahaha
For the keepin the soil wet well perlite and peat moss work well for retaining water but for the most part its healthy for the plant to dry out and get watered you dont want the soil wet all the time or else there's a chance that root rot might accure, plus the plant in veg state drinks ALOT of water once its in bud you will notice the consumption slows quite abit.
And for co2 well its great for plants, more co2 will help you yeild abit bigger and have a healthier plant, those home made co2 things work not bad you need like 3 or 4 of em too make it worth while but they have come out with co2 pucks for small grow rooms so maybe if you have a store near by that sells that sorta stuff you might wanna dry that if its not too much i'm haven't priced it yet but its less stinky then a fermenting bottle of yeast hahah
but you can also if your brave haha burn a propane heater or lamp in the grow room and the brings up the co2 quite abit but the heat goes up aswell so thats a choice you have to make depending on your grow room..
and last but not least, we breath in o2 and breath out co2 so we in general are co2 generators, if you just chill with your plants and blow on them and spend abit of time in the room it will add to the co2.