I would try to find a way without changing the approval engine for the Request table if possible. You never know in which update it would break ;).

Perhaps instead of removing the Workflow completely, you can just add a condition to check if your Request was created from the specific Order Guide and if yes, request an approval? The Order Guide field on sc_req_item would normally store which Order Guide it came from.

Then of course for the contained items, any WF you add would not have any approval steps. Requested Items will only be activated after the parent Request has been approved.