Is there a way to set a default sales order form per subsidiary when creating intercompany sales order from Manage Intercompany Sales Orders page? Our sales order forms per subsidiary have mandatory fields that cause the error. I think it uses the preferred one which also has its mandatory custom fields that are not populated in the intercompany PO.

You would probably have to set forms to roles within subsidiaries. Worflows can also help to set or unset mandatory fields based on user.