Share Spells: The wizard may cast a spell with a target of “You” on his familiar (as a touch spell) instead of on himself. A wizard may cast spells on his familiar even if the spells do not normally affect creatures of the familiar’s type (magical beast).
So yes, if you have the Share Spells ability then you can cast spells that normally do not affect the creature type. I believe every other companion that has the Share Spells feature has similar text.
With spell sharing can I cast enlarge person on my companion? Or do I have to wait for the later level spell specific to animals?
You can do it as long as you get the companion and the spell from the same class. If you get enlarge person as a wizard and the companion as a druid, you can't share it. If you have more than one class that grants a companion the spell can come from any of them.
EDIT: the above assumed that "companion" meant an actual Animal Companion (TM). If you meant a familiar the bolded part does not apply. I do not know why they differ in this regard, but they do.