Okay, I have a quick question: Assume I'm, for the sake of argument, a wizard with a familiar.
I then pick up the improved arcane bloodline feat for the serpentine bloodline, which gives me a snake familiar (Serpent friend) at sorcerer level -2 (which combines with the bloodline feat penalty to give it a total of -4 of my actual level as my effective wizard level for this effect).
What happens?
Familiars
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Levels of different classes that are entitled to familiars stack for the purpose of determining any familiar abilities that depend on the master's level.
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All familiars have special abilities (or impart abilities to their masters) depending on the master's combined level in classes that grant familiars, as shown on the table below. The abilities are cumulative.
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Arcane Bond (Su): At 1st level, you gain an arcane bond, as a wizard equal to your sorcerer level. Your sorcerer levels stack with any wizard levels you possess when determining the powers of your familiar or bonded object. This ability does not allow you to have both a familiar and a bonded item.
Serpent friend (Ex): At 3rd level, you can use speak with animals at will with reptilian animals (including various forms of dinosaurs, lizards, and other cold-blooded creatures), and you gain a viper familiar using your sorcerer level –2 as your effective wizard level.
Well... as always, it's a Storyteller call. Serpent friend has the Ex descriptor and doesn't have the same wording as the Arcane bond version of the Arcane bloodline. By RAW, it looks like they can have a normal familiar and a viper four levels lower. As an added plus, your snake will function in an antimagic field, where as most other familiars won't. :)
Another possible argument (one that I think no one will support) would be since "For purposes of using that power, treat your sorcerer level as equal to your character level – 2, even if you have levels in sorcerer" and "Levels of different classes that are entitled to familiars stack for the purpose of determining any familiar abilities that depend on the master's level" you'd combine your character level - 2 as your 'sorcerer level' and whatever level you had in something that grants you a familiar, like wizard or witch, thus counting (almost) twice.
The upside to this interpretation is that you'd get special abilities, intelligence, and natural armor faster, but other than that, it wouldn't change much, since the familiar's skills, hit points, saves, and base attack are still based on the master's actual numbers. This interpretation would make this application of eldritch heritage strictly better than boon companion for familiars as it seems to violate the level cap. Not being combat creatures, I'm not sure if it hurts anything.
Again though, this interpretation is offered partially in jest.
There are whole threads arguing about abilities based on character level stacking with abilities based on class level, and if it works.
I suppose then I need to read more, but truthfully I probably won't make the time.