Hello
Carrick,
Please find my response below for your concerns:
1. Remarketing and Similar audiences: A similar audiences list is created from a remarketing list with at least 1,000 cookies with enough similarity in search behavior to create a corresponding similar audience. Targeting Similar audience is currently not supported through the API.
2. Household Income categories: You may refer to the Income ranges listed here and their respective Criterion IDs to add them as targeting criteria.
3. Custom Intent audience: You will need to add keywords to use custom intent audiences. You may refer to this link for more details about custom intent audiences. You may use AdGroupCriterionService to add keywords with criteria type as CriterionCustomIntent.
Let me know if you have any further questions.
Regards,
Bharani, AdWords API Team