Edge select ?

You can load two objects and bake both. Personally though I would still recomend separating designing teh materials (Substance Designer) from applying the materials (Substance Painter) and use teh strengths of both applications

'why when using node : edge select i get dirty edges and glitches ?'
Did you bake a curvature map and what settings did you use? Also what settings in your node

'is it possible to use TEXELS in Designer?'

There is no unit as such for it. Designer creates procedural materials that are scalable and in which resolution can be changed. Maps can be directly output in maps up to 8kx8k or the procedural material itself passed to another application as sbsar e.g. Substance Painter and used at whatever resolution is required. How that translates to texel density on your model depends on your UV unwrap and the final resolution you choose when applying them to your objects.

do you have tutorials to learn how to use position map?

I don't know of a tutorial in Designer. You could take a look at a couple on Painter which would show you how a generator can use a position map and try and replicate the same in Designer. The principles are that you can use the map to give you the X Y and Z position of any point on th surface of your object (relative to a bounding box). That can then be used to drive nodes/masks to give effects that change with position or direction.

Dave