They're all the same species of fish, often it's a matter of collection location. There are really 3 color morphs, the lemonpeel, heraldi, and half-black, which are considered the A. pyroferus. These color morphs are the juvenile state, and I've never seen a good trace as to how each different fish changes into adult coloration. In the past, some people considered the half-black mimic as the A. chronixis, but I think now most consider it the A. pyroferus (similar to how the Blonde Naso and regular Naso are now both considered the Naso lituratus, though they used to be considered different species... only difference is collection location and color morph). The chocolate is just another common name which mostly refers to the heraldi mimic, though is often applied to any mimic tang (especially if they're sold as adult fish).
The eibli mimic is still considered a different species (or at least, last time I checked... as the A. tristis).