CosmoDB-多区域集合的事务级别

I have a USER collection which stores user information. Every item will these 3 fields:

-UserID
-Username
-Email

If in the future I need to make this collection available across different regions I understand there are 5 different consistency levels (strong, bounded staleness, session, consistent prefix, and eventual consistency). If I want to guarantee that two users in two different regions do not create the same user (with either the same userid or username or email) at the same time is my only option then strong consistency? A followup question, if I set this collection to be strong, do all my other collections also have to be strong or can I set the consistency level of different collections to different levels (For e.g USER collection is strong and POSTS are eventual)?