We've traditionally run a selection of development environments from a single cache instance using different namespaces for DEV TEST PREP etc. This has come with a few drawbacks, mainly that you then cant break down into namespaces per environment and everything is sharing various options.

Is there a massive overhead to running multiple cache instances from the same 'box' instead?

How best to share the memory setup in that situation, we usually pre-define around 80% for the instance, do we just share that evenly between the multi instance scenario?