Route Determination and Carrier

I have been assigned to a project in which I need to set up route determination for all customers. I have reviewed the documentation carefully, and understand the high-level configuration that is necessary. However, one item I cannot seem to find is how route determination can specify a carrier to be used for the shipment.

For the majority of our customers, we don't have specific routes with different legs, we simply have weight groups which determine the carrier we will use. For example, for customer ABC, if the weight of the delivery is between 1-20lbs, we will use Fedex Air, and if the weight is 20lbs+ we will use Fedex Ground. Another customer could have a totally different set of weight limits and carrier(s) they would like to use. We do have a few customers who would like to specify the specific physical route the shipment should take (east coast to midwest, then down to the south, or something like that).

For the first type (by weight) I understand that I can set up weight groups which will help derive the route, but I do not want the route to just equal the carrier. Instead, we would like to use route determination to help us determine both the route (physical legs in the trip [likely to be defaulted to a general route for most customers, but leaving the solution open for the future]), and the carrier, which would likely be placed as the Forwarding Agent.

Sorry for the long explanation, but if you have any suggestions, it would be greatly appreciated.

Thanks!