The agent must not be more recent than the server, e.g. an 1.6 agent will produce data that an 1.5 check engine will not be able to process.
An agent may be older than the check engine as there are compatability functions built into the check plugins.
I am not quite sure if an 1.5 site is able to work as a WATO remote for a 1.6 central site as the config file format has changed completely.
You will be better off to test 1.6 completely separate.