Your configs look good,essentiallypolicy-map type control IP_RULE1 class type control MATCH_PREPAID_SVC event credit-exhausted  1 service-policy type service name DOWN_DEF_TC1_V1 !will downgrade the service once credit is exhausted. You just want to ...

You might also placing appropriate ACLs and place your customers and other equipment in separate VRFs

ISG is supported on the 7200, ASR1k, C10k boxes. 7200 is the smallest

ISG control and service policy-maps look good.Could you provide the output of sh subscriber session de ? and the interface config ?