I have added 'ad hoc' kotlin files along existing java files in a large monolithic legacy application which is built with gradle (and some parts spawning shell, packer,docker, make ... to build). I dont think in your case build environment will be the issue -- although it can be confusing and inconsistent if you have cross dependencies between kotlin and java source in the same build. I have had very little problem -- barely needing to even glance at the Java Interop features.