If I have created a Visual Studio workflow for SharePoint 2013, should it be a Farm or Sandboxed solution? Can I create an App with workflow and associate it with the List on the host web?
The first thing is to look at the scope of your app. If it’s just for one site or site collection then sandboxed. If it is something accessible to the entire farm then farm based.
Next, you should be able to associate the sandboxed solution to the correct list without any problems. Then deploy and activate it to use the new workflow.