First, the Chromalux are no good for plants. These are incandescent bulbs and don't produce enough light. A single 400W metal halide or HPS would produce over five times as much light as your two 150W Chromalux. I would suggest that a single 400W metal halide or HPS would be suitable for lighting a 4'x4' area. Plenty for overwintering jungle tropicals, you could probably even do that with a 250W. Maybe you would want a bit more if you wanted to keep high light plants in growth.

Put them lamp anywhere you like, so long as it shines on the plants. A single lamp really needs to be above the plants to avoid shading problems and leaning growth. If you wanted to use half a dozen fluorescents instead, you could mount them vertically around the sides of the planting area. In all cases, you will need reflectors to get the light all going onto your plants.

Seedlings will do fine under 3000K, or the 3000K/6500K mix you have. No need to go to all 6500K. Avoid the actinic, they are too blue and the UV will fry your plants. Makes corals fluoresce nice though :)

What seeds are your growing? Things like annuals and other full sun plants should be grown with the fluorescents nearly touching the tops of the seedlings. Watch out for burning if you get the T5s too close. Use a fan or move them a couple of inches away from the leaves. Some tropicals would want less light, which you can get by moving them further away.

In terms of light available to the plants, metal halide and HPS are roughly equivalent. A strict calculation would show that HPS actually produces more usable photons than the metal halide. The problem with HPS is thar the colour can make the plants think it is time to stop growing and start flowering. If this happens with the plants you are growing then you need more blue light, and switching to a metal halide is one way to do it. Commercial growers tend to use HPS because it is more efficient and then supplement with just enough blue light to keep the plants growing instead of flowering.