Can we edit compliance rules or add our own?

Absolutely!  The rules in the Compliance Rules repository are yours to use, modify, delete, or augment as you see fit.  The Compliance Rule language (Gremlin) is non-proprietary and well documented.  Data Migrators can also assist you in adding or modifying rules, if you need it. 

Take a look at the MettleCI documentation to start modifying or writing your own Compliance Rules.