So from what i get reading your question, i would recommend reading more about container, compose files and how they work.
To your question, i assume when you are talking about adding to container you are actually referring to compose files (often called ‘stacks’)? Containers are basically almost no computational overhead.
I keep my services in extra compose files. Every service that needs a db gets a extra one. This helps to keep things simple and modular.
I need to upgrade a db from a service? -> i do just that and can leave everything else untouched.
Also, typically compose automatically creates a network where all the containing services of that stack communicate. Separating the compose files help to isolate them a little bit with the default settings.