Dyna-Gro is fine, it is less about the nutrient and more about the grower. Anything additional would be considered additives. I don't think you need to add anything, but ultimately its up to you. Personally I would use a magnesium sulfate additive,
Botanicare's Sweet is my personal option but General Hydroponic's Flora Nectar and Cutting Edge Solutions Sour Dee or Sugaree are also good options.
Since you are in soil I would also consider adding some beneficial fungi and bacteria. You can find Capulator's line of products here or there are plenty of good options at your local hydro store. I have used the Plant Success line with great results for several years now, they make
Great White, Soluble, Mike O'Rizey, Jellyfish and a few more. Then there is Roots Organic Oregonism,
Botanicare's ZHO and
Hydroguard and many others out there.
Other than those two types of products I wouldn't think you need much more right now as far as nutrient additives.
As for your use of space, in the 4 x 8 tent I would personally run 2 600's. This will give you even coverage over the whole tent. I would mark out two 3 x 3 boxes in the tent, leaving 6 inches around the outer perimeter and a foot in between each box. Center each 600 over the boxes. 5 plants will go in each box. This allows you to keep all your plants under the best quality and intensity light.
I am not sure how long you are vegging your plants, but maybe you want to spend a little less time vegging under the T-5 and give them a couple of weeks vegging under the HPS. You have tall plants with large spaces between buds; the transition to the HPS while still in veg may help keep them a bit shorter. You may also want to flip them to flowering a bit earlier. This will keep your plants a bit shorter. You may not increase your yield bit it will shave a few weeks of your grow cycle allowing you more harvests in the same amount of time.
OG's usually have a flowering time of 60 - 70 days, depending on variety and growers preference.