Quote - With regards Morphing Clothes - do those that use it copy all morphs from the intended wearer or only those morphs that you know/think will apply...?

That depends - Every added morph takes up memory. A CR2 can get unnecessarily bloated with every morph you could possibly throw onto it.

I keep a figure loaded with every morph and custom morph I have, then I use that as the base figure and pick and choose the morphs I think I will need. If I end up needing more, I will simply add them with Morphing Clothes. You can go back and forth, adding morphs, as necessary.

Quote - There is a way to reduce polygon count with some tools, for ex. Zbrush or Sculptris ? Or in some situation is better manually modify confoming cloth with zbrush ?

You shouldn't ever need to "decimate" (to reduce the poly count) a modelled piece of conforming cloth. In fact, that would be a very bad thing if it had included morphs in the CR2. (You might want to reduce the polygon count if you're using it for low-res, very distant renders, and need a lot of figures in one render, like for crowd scenes. But, otherwise, you don't want to do it.)

I'm sure ZBrush would be suitable for creating morphs in clothing to fit custom models. Sculptris would probably be fine, too, but I don't know how versatile it could be.

Don't be afraid of Poser's Magnets and Morph tool for adding morphs as needed to conforming cloth. But, if you need precise morphs or radical ones, the only real solution is to take it to a 3D modeling package. If I were you, I would start off with something cheaper than Zbrush, though. There are plenty of free modeling programs out there:

http://www.hongkiat.com/blog/25-free-3d-modelling-applications-you-should-not-miss/

In the Poser community, the most used free modelers are Blender, Wings and Animator, the latter two for their ease of use and length of time published, Blender for its powerful features.