In general, you should not be mixing the asset pipeline with Shakapacker and React on Rails.
If you're using React, then all of your CSS and images should be under either /app/javascript or
/client or wherever you want your client-side application.
If you are incrementally migrating a large application, your main concern will be how to minimize duplication of styles and images between your old application and the new one.
Please email [email protected] if you would be interested in helping to migrate a larger application.