All of the settings in the link you provided can be overridden, they exist in reference.conf as defaults. The Typesafe config library will merge your keys with the defaults, giving your's precedence. Contrived example:
If you want to override the default actions dispatcher to have a higher parallelism factor, you would add this to your application.conf:
play {
akka {
actor {
actions-dispatcher {
parallelism-factor = 5.0
}
}
}
}
All other Akka settings would stay the same, it just replaces the specific key that you overrode.