As for Tieflings having animal heads because some fiends do, I think it might be more of the nature of fiendish blood that makes that rarer in Tieflings, mortal blood attampts to rejects the Fiendish corruption. And I think for example in the case of a Tiefling of Glabrezu-heritage they far more likely to have an extra set of vestigial arms than look like a dog, because I think vestigial arms are simply more interesting.
For Guardinals, Cerdivals are often depicted as being Satyr-like they've generally been shown as the most Human looking of all Guardinals. Avorals have usually been shown as Humans with beak-like noses and feathers for hair. I don't see there being a good reason that the mortal descendant of an Avoral has an eagle head when many Avoral's don't have eagle heads, unless the Ardling/Aasimar was of Aarockocra, Owlin or Kenku descent.
As for how an Aardling or Aasimar, of Celestial Eladrin (aka Azata) background I feel they would look more fey-like and elemental (though I know that touches on actual Fey races and the Genasi), though maybe some plant-like features could be added in.