Hi, welcome to the forums
Rather than answering your questions directly I’m going to try to explain the differences between the extruders so you can appreciate why one is used over the other.
The way the standard extruder works is it “grabs” the filament by squeezing it between a hobbed bolt and a ball bearing and pushes it down towards the hot end. Most filament is rigid and can easily be pushed along. Pushing on flexible material is like pushing on a rope - it doesn’t want to go straight. If you look carefully at the extruder you will see a gap on either side of the bolt/bearing mechanism. This gap is the problem. With flexible material, it can bend at these gaps causing it to jam.
With the Flexystruder, the design is very similar. It still uses a hobbed bolt to grab and push the flexible material along. The biggest difference is that to get around the gaps the entire path of the filament is encased in a white PTFE tube. There is a notch drilled out to give just enough room for the hobbed bolt to touch the filament, but not enough room for anything else. This ensures that the flexible material doesn’t have anywhere else to go. In order to push the filament into the hobbed bolt to give it grip, a thumb screw is positioned on the outside of the PTFE tube to push the tube towards the bolt.
I’m sure this would make more sense if you looked at pictures. Unfortunately I’m going to be a little lazy and not hunt them down right now. Maybe you can find some or someone else will post a link.
Anyway, now that I’ve explained the differences between the two extruders and why flexible material doesn’t work in the standard extruder lets talk about why you shouldn’t use rigid filament in a flexystruder. By the way, it definitely does work. The problem is that the rigid filament is harder than the PTFE tube and acts like sandpaper where it rubs tightly (around the hobbed bolt). I haven’t tried it myself but I’ve heard that it doesn’t take long for the tube to degrade and need to be replaced. Replacing the tube involves using a drill press. It isn’t that hard, but I wouldn’t want to do it often.
So that’s standard extruders and flexystruders. The dual extruder just holds two standard extruders close to each other and the dually flexy has one of each. What makes them complicated is it is tricky to get them calibrated to each other. Also, in order to slice a model to print with multiple filaments requires a bit of work.
Regarding types of filament, as long as the TAZ can handle the temperature requirements it should work. Flexible material in the flexystruder and rigid filament in the standard extruder. If you want to use a flexy dually to combine flexible and rigid filament into a print you want to check out that the two filaments will bond to each other. For instance NinjaFlex works well with ABS but I don’t think it works well with PLA. I believe the extrusion temperature differences between the two is part of the reason.
Oh yeah, it is very easy to physically swap extruders. The only other thing you will likely need to do is to change the firmware on the printer. The Marlin firmware needs to be set up differently when the type of heater changes or the number of extruders changes.
I hope this all makes sense and I appologize if I dumbed it down too much.