Replace with Copper and "mixing" with Old Galvanized Pipe health concerns
It's really hard to read your question, but I think you are essentially asking: "Is it okay to replace galvanized pipe and connect it with copper?"

The answer is yes, but there are a couple things to keep in mind:

Copper and galvanized pipe will suffer from galvanic corrosion where they meet unless they are isolated. This requires use of a dielectric union 



Galvanized pipe also is generally cited has having a 40 to 50 year lifetime. Generally they will rust from the inside out, restricting the flow in the process. 



This restricts the flow, and is often the reason that older homes have such poor water pressure. It would be a shame if you get your new shower installed and then find out the pressure is awful, and it could very well be that it is the galvanized supply lines and not the shower itself.

There is also the possibility that there are metals leeching out into the water supply from this corrosion, but to know if this is happening for sure you'd have to get your water tested.

Once you start seeing corrosion on the outside of the pipes, it's just a matter of time until you have a leak. I've actually hard of some home insurance policies not covering damage from leaking galvanized pipes, so that might be worth checking out. 

There's also no reason you have to use copper, pex is a perfectly valid option, and for replacing a long in-wall run it's significantly easier to install than copper (it bends, and comes in spools up to hundreds of feet long).