Are you using ActiveState's PPM utility? It sounds like you tried downloading MinGW from somewhere and are trying to install it manually. If you're doing that, you need to make sure that it is compatible with the compiler that ActiveState used to compile their ActivePerl that you are using. By installing MinGW using the PPM utility, you'll definitely get the correct version of MinGW.
Would this impact any of my existing environment and occupy more space on my machine?
Following those steps, any new command prompts opened after modifying and saving the path environment variable will use the new value. Any command prompts that were previously opened before the change will not pick up the changed value for the path environment variable.
As for using more space, just simply changing an environment variables value shouldn't have any significant change to the space used. Installing an application will definitely use up disk space. The amount of disk space used will vary by application.