You are right that T8 and T12 is not compatible. I tried it (not knowing any better) and the T12 fixure make the T8 bulb flicker.

I am a newbie and I am just starting, but i am going to use 4 T8 bulbs over my lettuce. I am using 6500K bulbs.

There is a thing i discovered call Wien's law that lets you find out the peak wavelength of 6500K. Since most of the charts i have seen for the lights that plants like is listed in nanometers this conversion is handy.

http://greenpinelane.com/lights_main_menu.aspx

Wavelength (nanometers) = 3,000,000 / Col temp (Kelvin)

6500K has a peak of 462nM
4100K has a peak of 732nM

You can see 732nM is a bit out of range on the plant chart.

What i have learned is flourescent T12 and T8 tend to just not be powerful enough. You need more of them and you have to keep them close to the top of the plants. So if you have tall plants they dont penetrate deeply into the plant.

The T5's are more powerful and that is what you see in commercial grow lights.

This is what i have learned so far. Hopefully someone will correct me if i am wrong.

Here is a link that might be useful: Wien's Law

I have successfully overdriven T8 32watt bulbs for this purpose. This is especially good if you are pressed for space but need lots of light with low heat output. Everyone was saying how hard it was to reuse the plastic ends. But I figured out how to get out the wires to reuse my parts for this project. Once you setup your shoplight with dual electronic ballasts (one for each bulb), you get really bright lights for starting seeds. It voids the warranty on the ballasts, but only if you tell them what you are doing. And it shouldn't harm the ballast if you only overdrive the T8 32wat bult 2X. It is safe if you follow the diagrams and instructions. I used the Advance ICM2P32SC electronic ballast from my local electrical supply store. It is 110/220v. DO NOT try using magnetic ballasts!!! The trick to removing the wires from the end caps is as follows: Cut the wires back 1", then twist the sheathing until you can pull it from the solid copper wire, leaving just the solid copper wire in the end. Then, I used a radio shack 1/32" flat screwdriver (tinyest one in the set) to gently push the tab in enough to release the wire, while pulling the bare wire gently. Do not push too hard, or you will bend the tab inside and render it unusable. The wire should release and you can reuse the plastic end. I had really old T-12 shop lights, which had pcb laden ballasts(magnetic). And they always ran hot. When you replace the single ballast with dual electronic ballasts you overdrive the tubes. The 32Watt T8's were much brighter. The T-12's also work with the new config, and are brighter as well. If you use T12 fixtures for this project, you can alternate between T8 and T12 bulbs for whatever. This shortens the tubes life span, but tubes are cheap enough. My electrical supply store had the ballasts at 15.00 each, and tubes were only a couple of bucks each (T8). For under 100.00, I now have 3 flourescent fixtures, using 6 tubes, probably equivalent to a 250watt hps light, with far less heat. I have tons of T-12 bulbs leftover, so I'll overdrive them till I burn through them. This lighting setup works well for studio photography as well. You can get nice bright lights at the right temperature for film (Yes, I still shoot film, but produce 22megapixel images using a professional film scanner). I'm planning on a light box using these overdriven lamps after the garden is up. Who says you can't use flourescent lights for growing and photography?
The tubes get a bit warmer, so it is recommended to run a fan across them during your seed sprouting. Get that headstart on those beefsteak tomatoes and hot peppers!!

Here is a link that might be useful: Overdriving Flourescent Lamps

Nice article. I gotta try it. Also there are extra mounts in my units to add another ballast.
BTW I use a paper clip to remove wires from the sockets.