In short, YES. You see the vision. Welcome! 
Every part of Hanami is modular, both in terms of letting you swap things out of the full-stack framework, while letting users use libraries from Hanami that they like.
Tim gave a talk that touched on this, where he highlighted Roda, Phlex, and Shrine as Ruby gems that can be used across the ecosystem. I have some ideas about how to make this easier in Hanami by letting users easily create custom generators.
As for Brut and Yippee specifically, they both have different angles and visions of how they want to make Ruby web apps. This is good! They’re welcome to use any piece of Hanami that they’d like, but it’s also fine that they haven’t yet. We’ll support them if they do, and also. most importantly, support users who mix-and-match. (Specifically the forthcoming SQlite-only DB layer that will be part of Yippee is of great interest to me personally.)
Additionally, we want to support Rails libraries as well. In the spirit of welcoming all Ruby uses, and also specifically to provide a path for people who want to migrate from Rails incrementally.
Thanks for articulating this!