Any US citizen or legal permanent resident, living in the US, may become a financial co-sponsor if that person has enough financial resources.
You likely should review the instructions for the Form I-864 and Form I0864A. The forms and instructions are available for free at the government's website: http://www.uscis.gov/forms .
You likely will find that your fiance is not a "household member" as defined by the immigration regulations.
To reduce the chance of a delay to your petition because of an incorrect filing, you should review the instructions carefully or consult an attorney.