Are there any 6 limbed reptile fossils?
I think, you misidentified the unpaired anal fin of coelacanth as an extra pair of limbs.

I personally haven't ever heard of six-legged vertebrates, although the phenomenon of supernumerary legs is well known as an anomaly. Such anomalies can be easily induced in different vertebrates (including mice: Rutledge et al), if you treat their embrya with retinoid acid (which normally defines embryonic polarity) or fibroblast growth factor. In nature, different factors can lead to development of extra legs, and the most well-known and wide-spread cases are limb abnormalities in amphibians, induced by trematodes and pollution (including retinoid acid mimics?) (Blaustein &amp; Johnson).

That said, I would not be surprised by a discovery of a fossil amphibian with extra limbs. But such a find then should be interpreted as an abnormality, if not proved otherwise. It's true that the fore and the hind limbs are serial homologues, but their number seems to have been fixed very early (in contrast to the number of the fingers).