here is the thing I have heard about cool tubes.....and all this is going to be very debatable, I also want to note I have never used one so here it is. Cool tubes actually have less range of light and it is advisable to get one with wings that reflect the light out more. I hear the glass around the light actually decreases the reflectivity. A normal hooded light is going to give you better light and a wider angle. A lot of people will say a cooltube is more for controlling heat....and for most of us it is the biggest issue! For you with a 600 it surely would be an issue and since you are in a 2' wide and will prob utilize a 2'x4' area under the light you will be ok also take in consideration you don't have lots of verticle room. Sworth mentioned that before.....but note he is also using two to cover the length of the area. Cooltube will be less money as well. So you can't go wrong really. Remember the pot will take at least a foot, the plants will grow to 2-3 feet and you are going to need at least a foot on top, so you just used up all your height. Also you may be able to keep the light cool and be able to grow your plants right up to it but there is something called light saturation. Even if cool if you try and grow under 600W and the plants are less than 12" away you could burn them not by heat but over saturation.
I also use a passive intake setup, I actually used Panda Paper for the front of my closet behind the door. Installed with zipper so there would be no light leak and I could almost completely seal it. The negative pressure was a lot and wanted to suck in the panda paper if I had it zipped all the way down. So what i did was cut a 4" hole in it and attach some ducting so it could pull fresh air in. I ran it in and up toward the fan mounted in the cabinet, to blow fresh air and not let light in. That worked pretty well. The grow will not smell to bad until you get to the end of your grow. The filter works very well but some bud is going to be very strong smelling. Meaning anytime you open up your grow space smell will leak out. Again I am lucky because I can duct into atic which helps as well so I don't get a lot of smell. Where will you be taking the air from your cabinet and exhausting? That is going to make a difference. I had asked before where this cabinet will be located for many reasons. It will have a fan and be somewhat noisy, you need to pull fresh cooler air in and exhaust out. I am saying this again because if this was located in my bedroom (in my closet) or a room where people go in and out of it is not going to be as stealth as you had hoped. Again though getting the larger fan and running it half speed will help. I have a spare room no one occupies but if someone walked into that room they would notice the sound quite easily.
When I bought my 400 I was told it would do a great job for my space and could cover a 3'x3' area quite well. I was told 4 plants in my 2'x3' would work great each plant would receive 100 watts of light....you can get by with 50 per plant and still get a good yield. I would allow a square foot per plant....meaning every foot have a plant. In your setup I could see having 4-6 plants at most if you grew then normal, if you did a SOG even more. If you start topping and LST-ing and make them bushier even less. You can control how to utilize the area and thats a whole new topic. I recommend following Sworth on the SCROG method 2 or 3 plants will fill the entire screen in that space. I know you want multiple strains in there but l would take a trail run first. And if this is your first grow stick to the basics and see how they grow, fill the area, see how the heat rises and air circulation decreases as they get bigger etc. Once you get that down and have a predictable situation then start in with more plants, different strains etc.
I have been growing by clones and once they root they go strait into a 3 gallon smartpot from Veg to Flower. I usually veg for 4-5 weeks and get my plants anywhere from 12 to 15 inches and then flower ending up with 24 to 36 inch plants. They will grow 2 to 3 times in size when you go from Veg to flower. You really can't go above 24 inches in your box in my opinion and you may have to stick with Indica dominate strains (Sative like to grow and grow). I assume your starting with soil? Your close just have to think about all the little things that aren't so obvious. Room for fans, to work on plants for trimming, watering etc. I try not to move things around once in flowering just lock them in and let them go from there...