How can I manage multiple versions of Scala & SBT in my dev environment?

I suggest you take a look at paulp's excellent sbt-extras script. This will enable you to pick which version of sbt you want to use on a per-project basis, and sbt will allow you to pick which version(s) of Scala you want on a per-project basis.

I always use the latest sbt from Homebrew (OS X) and then control the Scala version and libraries from within the build.sbt.

More recently (Dec. 2016), you can try and check out sdkman.io, The Software Development Kit Manager.

It is a tool for managing parallel versions of multiple Software Development Kits on most Unix based systems. It provides a convenient Command Line Interface (CLI) and API for installing, switching, removing and listing Candidates.

It installs Software Development Kits for the JVM such as Java, Groovy, Scala, Kotlin and Ceylon. Activator, Ant, Gradle, Grails, Maven, SBT, Spring Boot, Vert.x and many others also supported.