Captain Pouchot, the French Commander at Fort Niagra had 486 men for a garrison:
Troupes de Terre: 149 men of the La Sarre, Royal Rousillon, Guienne and Pouchots' own Bearn Regiment. This last unit had actually rebuilt the fort between 1755 and 1757. I assume that they came in equal numbers from each regiment; the French practice was to create mixed detatchments to lower the risk of heavy losses to the parent unit.
Compagnies Franches de la Marine (Independant Companies of the Navy): 183 Officers and men.
Militia: 133 Officers and men - of the Detroit Militia? Can anyone help clarify this point?
Artillerymen: one Officer and 20 men. As to whether they were the royal or colonial artillery I do not know. Their small number must have meant that large numbers of troops must have been delegated to the guns' service. Dunnigan mentions that 75 fusiliers were assisting the gunners which figure I assume he derives from Pouchots' memoirs.
When Fort Little Niagra was abandoned and burned, the garrison made their way up the portage road to Fort Niagra. The Garrison at Fort little Niagra was two officers and 70 Marines and Militia strong. I would suggest that 20 of the garrison were Militia.
Pouchot also had two lake boats at his disposal.
They are described as being a brig and a schooner named the Outaoaise and the Iroquoise respectively. They were pierced both for ten twelve-pounder cannon, five to a broadside. I do not currently know their crew sizes.