dis-similar metals

I am replacing some old galvanizd water lines and behind the shower single handle faucet copper was run down about a foot to chromed unions which then joined to the old galvanized. Can I join the new copper from below to the steel unions. When is there a concern about dis-similar metals. I may be able to undo the unions but may not be able to take the top part out. Planned to use a threaded copper adaptor. Thanks - Don