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I like Scala, Spring, and Maven. Sbt is definitely questionable and I always use Maven instead. IDE support for Scala is not great but it's pretty close to good enough. I don't really like Spring Boot but I like Spring in general and use it for pretty much all projects.

After switching back and forth between Java and Scala a lot I find that I prefer Scala. I have never used it in teams of more than a few people though.

I would recommend the same thing I recommend to all teams. Mandatory code reviews. Don't think of them as judgement. Think of them as async pair programming.

If you want really bad IDE support you can always cross compile Java and Scala in same project with Maven.