I'm not an expert, but have a couple questions and insights...
I grow in soil, with no experience with hydro...but it appears to me like that wouldn't give any elbow room for 8 maturing plants. (12"W x 27"L x 10"H)

What kind of lights for the veg room? All light leaks sealed between veg and flower sections?

I'm a big fan of using more than one source of light, and would use 2 400w lights if heat can be exhausted out, but that's just my opinion. Lights can be adjusted, both in height and lateral seperation giving more flexibility to your growcab. Side benefit: If one bulb burns out, will still have one until a replacement is aquired.

One Aquamist for two growing rooms? Why divide into three rooms? Using anything on the floor to protect plants from cold concrete?

You'll have to wait for others to recommend nutes. I use Fox Farms nute system for soil, but they have stuff for hydro. Don't know how good their hydro products are, tho.