The best practice for SQL + NoSQL database design I’m in process of designing database architecture for my diploma project and I have second thoughts about different solutions. 

I know and understand all my objectives can be done within  relational database, but having in mind thousands of users using the app, where data changes dynamically, create the question: should I proceed with both sql and nosql database in parallel? 

So the app is based on relational db (sql server), but there are parts of the software where user can change data very dynamically: notes, handbook, kanban board, invoicing. What I worry more about than only changing the data, is creating new and deleting old (in that case there is no need to store duplicates of notes or revisions of invoices) and not ending up having tables with millions of records. 

Additional question: is it okay to store different users fragile data (like notes) in one table, based on FK(user_id)?

I would love to hear your comments on it based on your experience. I tried with good practices on google but with no big luck. 

Please advise or provide good book title for that matter.
From comments:

This question is incredibly open-ended and has an entire branch of the industry devoted to it, namely data-architects and program-architects. The best thing to do is to pick ONE concept and stick with it. Thousands of users is a low-threshold for performance, so any solution will work. Are you expected to present a design or an implementation? Any database system can implement almost any sort of ERD, so just focus on a solid ERD.
There are plenty of options on what should happen here, and there are professional patterns that you may miss in your design (knowing how to build appropriate M:N relations based on experience) or things like audit tables that you may not be considering (we do a lot of this sort of thing post-academia, and know a lot of questions to ask stakeholders to ensure we build the right things). Just do the best work you can, and work with your advisor to know if it's enough for your diploma project. - jcolebrand

And one other suggestion from another comment:

You might like to try using PostgreSQL which has both classic RDBMS and a Document Store capability within the same database - indeed, you can join between classic tables and JSONB documents. Just a thought! – Vérace