Thank you for posting your query on Microsoft Q&A. From above description I could understand that you are looking for advise on creation of Test B2C tenant and Test Application.
Please do let me know if you feel any difference in my understanding by responding in the comments section.
- As you already hold a subscription in you parent directory, you may create another B2C test tenant and link it with same subscription, this tenant would not impact your production B2C directory apart from billing as after 50000 authentications you would be billed on existing subscription.
- For your test application I would recommend creating a different test resource group and have you test web application, test Key Vault, test storage, test Private networks etc, hosted on the exiting parent directory. Creating a separate resource group and hosting all test services under same would ideally not create any impact on production environment.
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