Currently in 10.2 M020 there is no way to directly connect a promotion request soft type to a specific workflow without customization.
I believe this has changed since at least 9.0.
Today if you have specific soft types that introduce new attributes there is no way to directly force a specific workflow. This greatly reduces the capabilities of using soft types and can cause errors if you introduce new attributes in the different soft types.
What we typically have happen is the person will pick the wrong soft type, or the wrong workflow causing a failure. You should be able to limit the workflows based on which soft type was selected. This works differently from the rest of Windchill. If the OIR defines a Lifecycle, the lifecycle usually defines the workflow. Promotions allow you to give the end user an option as to which workflow to use, which is not always the best case scenario.
I think we should keep the multiple work flows because I can see a use case for multiple workflows for a specific soft type.
Here is a mockup I created for one possible method. This is the simplest way I think it could be implemented based on the existing way Promotion Requests are set up and other similar preferences. For each type you could define a default and available workflows in the preferences. You could define one workflow per type, or multiples with a default option.