Is it possible to have multiple gitlab-ci files in a single repo?
No, you can't have multiple gitlab-ci files per repository.
You can see more information in the following links : 


https://gitlab.com/gitlab-org/gitlab-foss/issues/18157
https://gitlab.com/gitlab-org/gitlab-foss/issues/28592


Now, there are some work in progress about this, but it's not available yet.
For example, you can see here ( https://gitlab.com/gitlab-org/gitlab-foss/issues/56214 ) that they are working on pipeline which can include other (complete) pipelines.

The include keyword is more about reducing the size of your main gitlab-ci file.