It's not "adding it to a workflow". You go to the workflow scheme and tell that to use your chosen workflow for that issue type. JIRA will migrate any existing issues when you commit the change.
I'm sorry, I don't understand. Issue assignments have nothing to do with configuring issue types ti use workflows.
The second part I think means you're worried about affecting other projects. Yes, you will, if the scheme is used by other projects. Although it only matters if the projects are using that issue type (that's handled by Gabrielle's answer)
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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