I just want to change the dock icons instead of system wide.
It it possible to only change the icons in the dock, and not elsewhere?
Use Candybar. It's free now as it's not as effective in Mountain Lion as it used to be.
Can you be more specific? If you mean add or remove new dock icons on Mac OS, this should work.
To Add icons: simply drag any icon from system onto the dock. This should add it automatically.
To Remove icons: just hold any icon which you want to remove and drag it onto the desktop and leave, this should remove them.