I'll leave this one for Mutt .... other than to observe
(a) Part 25 is an airworthiness standard and not generally applied retrospectively.
(b) Part 121, etc., are the operating standards and will dictate just what applies to your operation if you have N tail feathers.
(c) The draft AC is available if you ask for it but basically throws it back on the manufacturer to produce some useful numbers or procedures for the ops engineers to do the sums.