[GpsFrontier]

Sounds like you are planing quite a garden. First I would like to say I have some designs for using geothermal energy to cool the nutrient solution. These designs are in pdf format and the file size is too large for this forums pdf file size limit, so I cant attach them to the posts. But if anyone wants to send me a private message with their e-mail address I will be happy to e-mail them to you.

The nft channels would be made from 1x8's or 1x10's screwed together in a "U" shape, elevated by 4x4 posts and sloped towards the reservoir and lined with clear plastic to aid drainage.

I can see there being maintenance issues here. Lining wood with plastic, I cant see being very durable and the slightest thing can poke holes in the plastic. I would prefer to use plastic rain gutters, they are not expensive and will last much longer.

Inside the channels plants would be in grow bags.

I'm not sure if this is the best option for the roots in an NFT system. An NFT system supply's a constant supply of nutrient solution to dangling roots. With growing in bags there would be no dangling roots, so the whole bag will need to sit in the flow of nutrients. They will become waterlogged that way. Is this going to be a recovery system or non recovery system?

Then the channel will be covered with either a double layer of weed control fabric (which is black) or white/black plastic 6 mil draped over the top. There would be slits in the cover where the plants poke through only.

I don't really see a problem with this except if there were any issues that required you to take it off for maintenance. Then you may need to cut the slits all the way to work on the system, and replacing it might be a problem.

If I used the white 6mil it would keep roots cooler in the hot sun while reflecting sunlight back up to leaves.

You can always use both if funds allow, black on bottom and white on top.

I have 10x 2-gallon bags and 10x 3-gallon bags on order

these must be special bags because I am not sure why they would need to be ordered. I live in a small town but I can get bags of this size.

I am still learning about companion planting and I'll be trying it for the first time this year.

I think this is a big part of what you want ideas with. This is a good idea but unfortunately I don't have any experience with doing it, nor have I done any research on it. So I can't give any advice about it at this point. I would be interested to know more about it and how it works for you.

2. How far apart do the two channels have to be?

I am not sure if this relates to the companion planting or not, I don't know about that part. Although in general I would say they should be far enough apart so you are be able to get to each plant, but close enough to save space. There is always maintenance issues, switching plants, and unexpected maintenance that tends to come up. The easier you can make it on yourself later the better.

3. Because of the dolomite, is it ok to let the excess water from the tomatoes drain back to reservoir? Has anybody tried it?

I have never herd of dolomite myself, is this what you are referring to?

4. Should I go ahead with the NFT idea or should I go with my gut and have individual drip lines for each grow bag?

Each has its own problems to deal with. Depending on how you run them, clogging and uneven distribution can be a problem in a drip system. On the other hand using bags in an NFT system will become too waterlogged. The coco coir holds moisture very well. Constantly sitting in nutrient solution is a bad idea. You can possibility set it on a timer more like a flood and drain system to give the coco coir a chance to drain.

5. Is a 3-gallon bag big enough for a tomato? Bell Pepper? Zucchini?

3 gallon root space for peppers should not be any problem. Tomatoes and zucchini not so sure, I guess it would depend on the variety and how long you plan to grow them. As well as how big you would want them to get.

6. The sun was so brutal last year, what shade cloth should I use and when should I put it up (don't want to stunt growth but last year tomatoes and peppers just shut down production till it cooled back down again)?

The shade cloth does not cool the weather, but can/will help with rising temperatures resulting from direct sunlight on the plants and hydroponic system parts. Keep the root zone between 68 and 72 degrees at all times. The plants themselves mostly depend on the variety of plant. Some are heat tolerant versions and will be able to withstand more heat. If the idea is simply cooling the plants and system by blocking light with shade cloth, I would not recommend using a thick shade cloth. The thicker shade cloth blocks more light rays, but I don't think it will do any more good with regards to cooling the plants and will deprive the plants from the light.

7. Heavy rains last year nearly killed everything, any thoughts on how to protect the plants (the black/white 6mil plastic may help???)

Grow inside? Not sure I could control mother nature. I am not sure how big of an area you are needing to protect, but you can certainly cover it with plastic before the rains hit and uncover it after. Though the winds may be a problem because they will hit the plastic like a sail, and can do more damage than good if not done right.

8. On the patio garden, which plants would need to be in self-watering containers and which can stand a bit more variability in their hydration?

I am a bit unclear hear. How exactly are they being self watered (are you talking about a wick system), and what growing medium are you planing to use for them?