Hi,
You can define CAR or policing using MQC and apply it on the internet sub-interface in the CE router.
Using CAR:
1.Define ACL first. Here you can say match any as the entire internet needs to be rate limited
2.Configure this "rate-limit input/output conform-action transmit exceed-action drop under the sub-interface.
refer this for CAR: http://www.cisco.com/en/US/docs/ios/11_1/feature/guide/CAR.html
Alternately you can apply this on the interface where Fortigate is connected for both inbound and outbound. The exceed action is usually drop which you can mark down the IP Precedece/DSCP as well.
HTH
Arun