Does FAMUNIT include individual families (i.e., families with only one individual) also?

It seems weired to count an individual as a family… American FactFinder defines families with size two or more. Same thing on IPUMS (ACS)?

Yes, FAMUNITincludes families with only one individual. Note, that you can adjust this definition to however best fits your needs upon downloading IPUMS USA data. For example, you can define families as needing two or more individuals. The FAMSIZEvariable may be helpful for this purpose.