I may have missed it but I was trying to figure out what a class only having a single sphere of influence means. Is it
a. That class AND the base species only have abilities from a single sphere
or
b. That class only has abilities from a single sphere but the base species can have abilities from other spheres (meaning any class that takes only a single trait from a single sphere automatically qualifies)
Also, a follow up question, I would assume when making a new class from an old class if the new class added traits from sphere A to a class that was taking abilities from sphere B that the new class would not qualify for the single sphere bonus as it has sphere traits from 2 different spheres even though the new class only added traits from a single sphere.
It really comes down to does the model have more than one Sphere of Influence. If it does, then it cannot get the bonus.
If the model has a trait from some other domain like a Lineage or evolutionary branch- these are not Spheres of Influence. So you can get or keep these bonuses that way as well.
Branches of Life- There are different types found within the Domains.
Spheres of Influence, Lineages, Evolutionary Branches, Classifications
A is correct, to obtain the single sphere bonus both the species and the class traits must be taken from a single sphere. Incarnate Fey Sub-Classes obtain this bonus and may be the way you want to go if you want a class to receive the bonus. Incarnates are built using a single sphere on influence and remove all other traits that are outside that one, including the ones taken for your species.
As for your follow up question, Ravenous Miniature's is correct.
I'll take a stab at being helpful.
Part B of your first question would be right as long as that class has no other traits you'll get the benifit.
If you look at the core rules both the Fey faction and Bieast(Monsters of myth and legend) faction are treated that way.
For your follow up question
Since traits from your previous class takes up class maximum traits from the new class, then the new class wouldn't get the benifit of single sphere if they have more then the one sphere.