I am doing the same thing, but using them as Tyranids, since there are more unit equivalents (Stegs=Fexes, Suarus=Genestealers, Skinks=Guants, etc.). As pointed out, it's easier to go from organic to organic, then to tech.
I have, however, seen some cool "Dino-rider" Guard conversions, even some Tau Lizardmen. Try a Google search.