You can have a look to this french website dedicated to flyfishing (and no-kill!):
peche mouche forum mouche pche a la mouche - Gobages.com
Many guys here, including webmasters, live just by the Dordogne.
As far as I know, the upper part, just downstream of the last big dam and for about 80km are very well known for trout and grayling even if nearly all fresh water species can be expected there (barble, chub, dace, pike, perch, sander, gudgeon, carp, roach and so on). One tributary called Maronne has also an excellent reputation. These rivers are to be fished with yellow duns, miscellaneous sedges, hare's ear, CdC, gold beaded nymphs... Wading, long drifts with thin lines and long tippets are the rule.
Anyway, you will have to be very carefull about the frequent and dramatic change of water level due to electric power stations... Try to contact the above mentioned guys and may be they can help to provide you indication about this safety matter. Many canoëing in summer also...