kodak, checking back in on you.
do you have an understanding of what an auto flowering period, and a photo period is all about yet?
auto flowering is a plant not needing 12 hours of darkness. you can leave light on it 24/7 from start to finish, and it will be happy. this is what i would suggest you do as a first time grower, then if you enjoy growing auto strains, you can learn more about them later down the road as to giving them 18/6 light/dark.
photo period is a plant that requires 12 hours of constant light, and 12 hours of uninterupted dark to flower the plants.
these are two separate plants that require separate lighting needs, and is wise not to run them togeather under the same light. not totally impossible, just not good to try and jump into being new imo.
i would had suggested keeping the 400 being a new grower that is doing a first grow.(you start at the bottom in any good building project)
heat is going to be your main issue with that 600 in the tent. pay close attention to venting that heat out now. thats going to need to be your main point of research now you will want to study up on imo. put a small thermometer in your tent also, so you know exactly what your working with on venting the added heat out. (heat out, fresh air in). plants need lots of fresh air. venting out the heat in a tent grow will be your #1 concern...it is the most important aspect of a tent grow. (your ability to keep temp under 80f will dictate your success).
i would suggest a test run with bag seed to give you a general layout of how a grow works, before burning up your money seeds. but yeah, venting is #1 aspect to being a successful tent grower imo.
were here to help...no question is a dumb one...we all went through the start up stage, and the figuring out part of how to grow our own smoke. so no matter what the question may be, we want to see you become efficient at growing, and were here to assist, so ask away anytime.
peace...