First off, the player in my game who brought these questions up said he already searched around and couldn't find any answers, so I'm taking him at his word as I post this. So if the answer already exists, I apologize.
1. Can a shaman take improved familiar to augment his spirit animal?
2.How does an animal spirit change when the shaman gains the true spirit ability.
animal spirit reads
By communing with the incredible powers of her spirit, the shaman forges a cherished bond with one specific servant of that spirit—known as a spirit animal. A spirit animal is a creature chosen by a shaman to serve as a conduit, allowing her to more fully access the magic of her spirit on a daily basis. The shaman’s spirit animal also grants her special powers. This ability uses the same rules as the wizard’s arcane bond class feature and is treated as a familiar, except as noted below.
A shaman uses her level as her effective wizard level when determining the abilities of her spirit animal. A shaman can select any familiar available to wizards to serve as her spirit animal, although her spirit animal is augmented by the power of her chosen spirit. Once selected, the spirit animal cannot be changed. Although a shaman’s spirit animal uses the statistics of a specific animal, it is treated as an outsider with the native subtype for the purposes of spells and abilities that affect it.
The bold part would make me think improved familiar has no interaction with this ability. Or does this mean the appearance of the animal spirit remains unchanged, but it can still gains the abilities of the improved familiar.
true spirit ability reads
Companion Animal (Su): The shaman’s spirit animal takes the form of an animal companion of her choice, using her shaman level as her effective druid level. The animal retains all the special abilities and the Intelligence score of the spirit animal, but also has the statistics and abilities of an animal companion. If the animal is dismissed, is lost, or dies, it can be replaced in the same way as a normal spirit animal.
How the above reads to me, the spirit animal retains it's int score, and still gains any benefit listed in the special ability column of familiar, but everything else is based off the animal companion table, including hit points, saves, atks, etc. It would lose the familiar armor class adjustment, for example.
Someone mentioned a familiar always has half the hit points of its master, but the wording above felt like it would supersede that rule.