Now everything is clear! I also tried REX 2.0 weather engine alone and it is worse than ASE: loading textures lasts much longer than SS mechanism found in ASE even if clouds and sky textures are modified alone, working with FSX is not so smooth (WASys - what is it???). So the right way is to choose best texture themes in REX to make appropriate SS in ASE. Later REX can go to bed and ASE takes care of everything! Maybe this is the most expensive option but worth every penny. Tomorrow I will fine tune my several themes in REX and bring this story to happy end.

Excellent!