First of all, I agree entirely with your comments on Chartrand and his Osprey books are a very good place to start in studying the French émigré units. He notes his debt to Grouvel (link to these on La Sabretache's website already provided) and to C.T. Atkinson's articles in the Journal of the Society for Army Historical Research (Vols XXI and XXII) - given the constraints of the Osprey format, these give much more detail and are well-worth searching out.

There is a biography of Phelippeax - "Echec à Bonaparte : Louis Edmond de Phelippeaux, 1767-1799" by Ghislain de Diesbach and Robert Grouvel, Perrin, 1979 which I have not read (I intend to look at it the next time I go to the British Library) which should fill in the background. As far as I know, he served in Conde's army and was then involved in stirring up Royalist opposition to the Republic and had some interesting adventures involving escapes from French prisons, rather than coming through the Engineer cadres you mention. I had a look in the London Gazette for details of his British army commission, but couldn't find anything (may be difference in spelling) - let's see what the biography throws up