If I set off (no HD texures) this settings is applied to all FSDream Sceneries installed in my PC or is it possible select only one scenery and let the others in HD resolution? 
That's not an "FSDT setting". As you can see, that option is indicated together in the group of FSX tweaks that are applied globally. Everything that comes with textures larger than 1024x1024 ( scenery, airplane liveries or VC textures ) will be affected by that setting at the same time.
It's not an "Addon Manager" setting, it's an FSX setting, the Addon Manager just give you the benefit that you can set it and see the result immediately, instead of restart FSX, which is what you had to do if you just changed the FSX.CFG manually.
Which also replies to your question: since you can apply the setting without restarting, by changing the setting mid-flight, you are effectively adapting the resolution per-scenery.
