Can we have per service, policy based results validators?

Yes... you can.

You need to implement org.apache.rampart.PolicyValidatorCallbackHandler interface and add the following configuration to your service policy - RampartConfig.

<ramp:RampartConfig xmlns:ramp="http://ws.apache.org/rampart/policy">
  <ramp:policyValidatorCbClass>my.CustomPolicyValidator</ramp:policyValidatorCbClass>
</ramp:RampartConfig>

This PolicyValidatorCallbackHandler interface allows different implementations of policy based results validation. Default implementation is org.apache.rampart.PolicyBasedResultsValidator.

It's always recommended that you use the default PolicyBasedResultsValidator - if you do not have a very specific custom requirement.