Assertions for Load Tests

Hi,

I am testing traffic shaping and I am trying to determine the best type of assertion to use in the load test module. An example of a requirement is that web service "A" will only accept 6 transactions per minute and all others will be delayed for the remainder of the minute. I want to have an assertion that fails if a 7th transaction is transmitted within the 60 seconds I've specified in the load test parameter. Any suggestions?

Thanks,
Matt