Stick with HQI ballasts for running DE bulbs ( I would even pick that over an Icecap or other e-ballast, so no worries there)... thats what they are rated for. Operating them at a lesser output will mean they wear out faster, and cen be just as bad as overpowering them. Doing HQI's w/ 10,000K DE's will also mean you will more then likely get a good two years out of your halides, where if you were running 400s that were bluer, you would barely get a year out of them.
Go with any DE reflector that does well. The reef optics3 from SLS, lumenmax2, or my pick which would be the lumenarc DE fixture. In the past, the lumenarcs have edged out the lumenmax's in performance, and with the lumenmax's, you are paying an extra $50 ea for the black enclosure. With the lumenarc DE's you dont get the black box and save some $$$ (I assume you are not going to have the lights visible due to the size/scope of the project). You can get the lumenarcDE here for $109 on sale...
http://www.reefexotics.com/lumenarc.htm
The T5s have plenty of punch... a linear bulb will penetrate better than a point source will.
As for how many, well... you have a large tank. If you look at large tanks like Energy's, and many Japanese reefs, you will see a trend where they simply light the center, or simply 'spotlight' the areas where the corals are and leave the rest darker. This lets them have the light where they want it, and off the glass, which means less wattage used, and less cleaning algae on the glass. You might consider it with a tank your size. There is a 8'x8'x36"h tank in Chicago that uses a similar concept, but we put a skylight in over it, so it really didnt matter so much. But, we did make a 12" parimeter of T5 lights around the outside. This parimieter hasnt been used yet, as the 6'x6' skylight has been plenty so far. But this parimeter does keep alot of light off of the glass, so cleaning so far has been minimal. The T5s are even angled inwards to that the light they produce when they do get turned on will be angled inwards.
You might consider this. Where and IF you place lights depends on what you want to do with them. If you are having a central overflow, then that means you can leave out a light in that area...
Without knowing what are the viewing sides (is it 360?), I can give a cookie-cutter suggestion and you can take it from there what to move slightly or leave out. I would start with 5 250wattDE's in a cross pattern... 3wide, 3 long, across the 7' and 6' lengths of your tank, leaving the corners dark until you are sure you need them. For initial setup and until your corals need light in the corners, what's the point, right? Then, add these 4 halides in at the corners, for a 3x3 grid of 9 halides. At this point, you might also be able to remove the center halide, as you may not notice since it will have lights all around... so 8 halides in a 3x3 grid with the central one missing.
As for the T5s, the system I have seen that works best is to use a 2:1 wattage ratio of halide to T5s (blue and actinic T5s). So for every 250wattDE, use 2x54wattT5s. I would stick with 4' T5s for this, staggering them across the tank. This should mean you use about 20 T5 bulbs... not too bad really. You could do this with strategic placement of 8 and 4 bulb tek fixtures if you wanted to. Or, you could stick with just 5 of the halides, and use more T5s (sun and aquablue daylight bulbs) to fill in rather than 4 more halides. I can assure you that if you substituted 4x250watt halides with 1000watts of T5, the tank would be much brighter, heck, even just 16 more T5 bulbs (864watts) would be brighter.
I should also add that the 2:1 wattage ratio is based on 1/2 the T5s being blue+ bulbs, and 1/2 of them being actinic03s. The blue+ bulbs have almost the same intensity as many daylight (aquablue) bulbs, about 4x that of the actinics. Now, most 10,000Ks have loads of actinic already, as much or more than many 20,000K bulbs (you just dont see it because of all the other spectrums). So the actinics are somewhat for looks, and you COULD reduce the ratio of T5s needed if you decided to simply go with blue+ bulbs. The look would be very similar, and in most areas, you wouldnt even notice, but I can respect when people want actinics because of the 'pop' it can add. Perhaps using less actinics, or strategic placement for viewing would also aid with this. But if you were to use only blue+ bulbs (or minimal actinics rather than 1:1), then you could cut down from 20 T5s to 14 with the 9 halides (or 30 54wattT5s with 5 halides).
Now dont get me wrong, this isnt the 'deep blue sea' look of many 20,000Ks, but more of a 'dayblue'. Not 14,000K either. Its very blue, but has plenty of warmer spectrums as well. And like I said, corals love it. This combo makes corals look better than anything I have seen, and fast. You know how stony corals grown under T5s seem to have a glowing pastel look? And SPS grown under halides get dark and intense, but not all over? Combine those two into one and you get what Im talking about. Its unreal, and I cant seem to replicate that growth or color under any of my tanks that all happen to be either all T5 or all halide lit. So my next tank is going to be this 2:1 ratio as well.
Now even though this setup wouldnt give you the 'deep blue sea' look like all radiums/20,000Ks or something, its still very blue. BUT, all you have to do is shut off the halides, and leave the T5s on, and you get a '20,000 leagues' look of all bright purple and blue... more like a 35,000K look, lol. So you still get the blue look.
Also, this suggestion is pretty cookie-cutter. I based it on a 6'x6' grid that is 30" tall, but the extra foot for the 7' ends isnt much at all. I was just at a friends place yesterday who runs a 225g (6'x2'x30" tall) with 3x250wattDE's and 4x39wattT5s. He's using pheonix bulbs (which actually put out more than Ushio 10,000Ks) and actinic bulbs for the T5s (pheonix bulbs have tons of blue, but lack actinic), and he's able to grow anything he wants. And you would be looking at more light that this even because you would have 2x as many T5s, and be using 10,000Ks with even higher outputs than the pheonix 14,000Ks.
