A substantial number of folks use Metals (my anecdotal impression is about 10%, although I haven’t seen any concrete numbers recently), although it’s true that IntelliJ is still dominant. That said:
That’s generally a non-issue. IntelliJ and Metals are just the editors – they are absolutely required to conform to the language, and they don’t make any changes to it. The project as a whole should not be IntelliJ-based, it should be sbt-based: IntelliJ imports the sbt declarations and builds the IntelliJ environment based on that. It is rare (and usually discouraged) to even check in the IntelliJ files.
(I believe it is technically possible to build a Scala project in pure IntelliJ without sbt, but that would be regarded as pretty weird by most of us. In practice, I’ve never seen anyone except a beginner try to do so. Sbt is, in practice, the primary project-definition language. There are alternatives like Mill, or occasionally things like Gradle, but I don’t recall ever seeing a non-trivial project built primarily in IntelliJ.)
Obviously, if you have editor customizations, those probably don’t port from one to the other, but that’s usually a personal preference, not a project-level consideration. Even formatting rules are usually defined in a standard tool like scalafmt, and imported from there into IntelliJ.
As far as I know, it’s not terribly unusual for a given project to have some folks on IntelliJ and others on Metals – the environments have different pros and cons, so people have different preferences. AFAIK, if you’re following standard Scala best practices, there shouldn’t be any difficulty.