I have a few questions: 1) Is there needed a workflow model for the form identified using process ID? Let's consider adhoc workflow - I cannot find workflow model for it? Is there some default workflow model type for it? 2) How to associate form with content type?
There is workflow model for each and every workflow.
Task defined in model are mapped with task defined in process definition file though task id. In case you are creating you new workflow and reusing some of the out of box workflow task you may need not to add those task in your workflow model as they are already define in workflowModel.xml file.
2) How to associate form with content type? How exactly you want to associate it? Because content of any type can part of any workflow irrespective of its type.