this is the right answer

Well the easy way to diagnose that is to run the car full-speed from one end of a parking lot to the other, and then turn it around and run it full-speed back to you again. If it leans-out or if the temperature reading immediately after the run is too high, you know the HSN is too lean.

Or to put it differently: If you can't run your engine at redline for at least ten seconds without misbehavior, it's too lean.

That's a nice myth you've got there. I bet that helps OS sell a lot of glowplugs.

You can change the brand of glowplug any time you want, you just have to re-tune the engine afterwards. Same as if you change any other part of the engine.

That's not true. Not all turbo glow plugs fit in all engines, just try to put a Nova plug in a modern Picco and the engine will have a leak, if you tighten a little bit you'll get a nice head button broken (a $40.00 mistake)

That has nothing to to with the taper.... Novarossi is the only one having the M8x0.75 thread running up to the start of the taper while all other plugs have the thread stopping 2mm before the end and some have even a smaller end tip.
Putting a novarossi plug into another brand engine you need to use some force to screw it up to the end and what you are actually are doing is screwing a part with thread into a part w/o thread and that will damage a part of the sealing construction.

So far I have seen can any other plug be used in any engine.

I've never talked about the taper. Just said that (as you explained) putting a Nova plug on a Picco will damage the sealing leading to a leak. Right?
I know that for first hand because my brother did it in front of me even when I told him not to. He ended buying a new head button.

Okay, well, obviously if the glowplug design is proprietary then you have to use just that one kind, but that's not what James73 said originally. He said once you switch to OS-brand glowplugs you have to keep using them. You don't. That's like people who say you have to keep using the same-percentage nitro fuel for the life of the engine; you don't have to do that either, you just have to re-tune the carb and re-shim the cylinder head.

Basically, any rule that states "once you do X you have to keep doing X forever" is just a way to avoid having to make multiple secondary changes to compensate for the primary change you made first.