There are so many things to consider when planning a grow room. How stealthy do you need to be (smell, etc.)? How large are the 12 plants? What is your climate like, and is the room well-insulated?
My first suggestion would be that 600W (1000W with the two bulbs) is not nearly enough light for that many plants. In my first grow, I put three plants under a 600W HPS, and it was not enough. A 3x3 area is about the best you can expect to cover and still get decent quality bud. Adding the MH will help, but you are still limited to an area of about 4x4 (maybe 4x5.7 if you paint the walls bright flat white to help reflect light). Ventilation is very important. My Flowering room has about 50% more volume than yours, and the heat from 1000W lighting is easily controlled with a single 6" blower at around 400CFM. Just keep in mind that it will suck the heat from the rest of your house in the winter and pull in outdoor heat in the summer.
The fact that you have drawn up a plan is a good sign. Do a lot of research. Then, do a lot more research. For every detail you must consider, there are hundreds of forum guys who have dealt with the same situation.
