I'm not sure that I follow. What do you mean by "base form"? The way the ability reads the familiar is effectively both a familiar and an animal companion gaining all the abilities of both.
It doesn't say 'can' or anything like that, it just says that it 'takes the form of an animal companion' which I read as there being no choice.
A more interesting question to me is whether the animal companion/familiar can keep familiar archetypes - Valet, for example. Sharing teamwork feats with your animal companion is typically much more useful than doing so with your familiar.
A more interesting question to me is whether the animal companion/familiar can keep familiar archetypes - Valet, for example. Sharing teamwork feats with your animal companion is typically much more useful than doing so with your familiar.
seems like it would, it says
The animal retains all the special abilities and the Intelligence score of the spirit animal
so, it gets to keep whatever it had regardless how it got it. In theory you could also take animal companion archetypes. So, you could take the Feytouched archetype so that it at least will be smaller, thanks to the "small but fierce" trait.
The animal retains all the special abilities and the Intelligence score of the spirit animal
For extra Argument-From-Ambiguity, take the Improved Familiar feat and try to keep your Arbiter (Inevitable)'s Regeneration(Chaotic) abiliy.
But then, since this is a capstone ability, that's not really too terribly over-the-top...