Workflow Start variables can be changed by opening the workflow in the Nintex Workflow designer and going to workflow settings > Variables.
To export a task form open the workflow in the Nintex Workflow designer. Find the Task that has the task form that you want to re-use and open its configuration. From the ribbon click the edit task form option to display the form. Click export from the ribbon.
To reuse the form do the same as above on a new Task action but click Import instead of export and select the form you just exported. The new task will now have the same task form as the one you exported from.