If I wanted to launch a workflow ONLY during a forms process (rather than through a starting rule), would I just disable the starting rules of the workflow, and call the workflow from the forms workflow service task?
Yep, that is exactly how you would do it.
Simple answer: yes.
More elaborate answer: The workflow can still be triggered from forms even if it has starting rules enabled. However, you may want to consider creating a copy of your workflow, without any starting rules, to work only with the forms process, and leave the original workflow with starting rules as an independent workflow.
Thank you both for your quick responses 
. I used to have the starting rules enabled, and I'm guessing because the rules were so generic and there was no error handling, this is what triggered a mass email event to occur when a single form was submitted. This workflow will only launch from a forms process anyways, so I am totally fine with locking it down by disabling the starting rules.