I know it's probably dumb question, but I didn't notice explanation for it anywhere. Anyway, how to create datum plane symmetricaly between two planes/surfaces? Is there direct procedure or I have to create datum points, axes etc. first?
If you know that the datum plane always will be symmetrical, then you can use relations. I.E. x is the distance between the planes and y=x/2 is the distance of the symmetrical plane from one of the planes. This way the plane always will be symmetrical. I suppose you know how to work with relations.
There is no midplane pick. You have to do a relation.
Another option is to create a feature using the both sides attribute and the blind depth option. This will create the feature symmetrically about the sketching plane. This may not fit your particular situation, but it works nevertheless.