As I have pointed out (with backup), French artillery and engineer officers were engaged in the forces of the Duke of York and Lord Moira in the 1790's. As a further piece of evidence from the National Archives, Moira had 2 French engineer officers with his forces intended to support the French Royalists in the West of France in November 1793 - Messrs Chatigner and Verban - Moira proposed that they "be paid as English Engineers holding the rank of Captain" (TNA: WO 1/174 p. 30)

As for the French officers with Smith at Acre, Shankland's "Beware of Heroes" states that "His [Smith's] Royalist friends were enrolled in the Turkish army" (p.50), which is why I asked for references for them holding commissions in the British army or navy