had thought for a long time they might be able to convert submarines to their own use (since they were already in the Med as against the journey of their own u-boats there past Gibraltar?) but @Carl Schwamberger convinced me otherwise for practical reasons he has reiterated in this thread.
the most plausible ships were merchant ships, easy (easier) to arm with standard German guns and operate. would not discount the value of KM having some of the faster transports prior to Nov. '42
if the Germans had pursued collaboration they might have allowed French to use their super destroyers around Syria
On the other hand the 6 destroyers would have been a useful addition to the Axis naval strength in the Mediterranean. Firstly because the Italian destroyer building didn't keep up with losses and second because they were the fast and heavily armed large destroyers of the contre-torpilleur type.
IIRC from the British official history of the war in the Mediterranean and Middle East it was touch and go whether Vichy France would declare war on Great Britain.
Another thing - if the French ships in Algeria get handed over to the Axis in some capacity or worse Vichy France joins the Axis, I would expect the US to move on French colonies in the western hemisphere very quickly.
strange as it sounds a "better" result for the RN might be best case for the Axis? if the Dunkerque was total loss in harbor and Strasbourg sunk while fleeing? a forced scuttling of French ships at Alexandria?
the resulting outrage (and loss of their main bargaining chips) might result in greater collaboration?