Hello jamromhem,
Sounds like you have been planning to build your hydroponic systems for a while now. I'm having a hard time picturing exactly what you have in mind, as far as the vertical setup (X verses H). But just thought I would mention if you are looking for a less expensive alternative to the square vinyl fence post tubing, but still would like larger than 4 inch PVC or irrigation tubing. Check with local pluming supply houses, and or water utility supplier. The pluming supply houses tend to stock larger PVC tubing, and some will even cut it for you, and they usually sell it by the foot. I got some 6 inch schedule 40 PVC through a local pluming supply shop for $1.78 a foot, and they cut me a 4 foot section. I recently found out (today) that water utility suppliers carry larger pipe because they carry the sewer supply's. I was looking for 18 to 24 inch wide PVC at the time, that I plan to use as part of a subterranean heating and cooling system I plan to install for a greenhouse. But I also have plans to build a vertical tube
hydroponic system when I can afford it, that the larger tubing is needed for too.
As for containing the roots, they tend to grow larger than most people think, and they'll grow down any tubes that have water in them. Without really being able to picture how you plan your setup. I would say just expect the roots to get in the tubing, but construct the system so that cleaning the roots out of them will be easy as possible. I call it building the system with easy maintenance in mind. But I think if you plan to grow tomatoes in a DWC system, it was a very good idea to make it a recirculating system.
The lettuce and herb setups you plan to build sound fairly straight forward in design, and it would be best to grow them in separate systems as you have already planed to do. The garlic and carrots will be interesting, I haven't grown them myself, and I haven't really seen anyone else do it in hydro. So it will be interesting to see how that works out for you. Garlic is on my list of things to grow in hydro someday, however carrots are usually fairly cheep here. It will be cool to see the pictures of your systems, I always like to see more of other peoples hydroponic systems. Take lots if you can, even while you construct them while your building them.