If you are going to have a co-sponsor then you may file for a fee-waiver application but make sure to write down in there that you will have a joint-sponsor for your adjustment of application. Also, you will need to submit/prove how you guys are supporting yourself in the light of being an indigent person. And if you will tell that someone else supporting you, which might be the case, then make sure to send the income tax returns of that person for the last two years, otherwise USCIS will ask those documents later on if you will prefer USCIS to consider your fee-waiver application.
I think you have nothing to lose by filing a waiver application since you are going to use a joint sponsor anyway for your AOS. All you will lose is time as it takes 1-3 months for a fee-waiver application to be adjudicated unless more documents are requested for further consideration. When USCIS states that it's not advisable to file a fee-waiver application for applicants for AOS then they were more concerned about public charge ground for AOS to be denied but when someone is going to use a joint sponsor then this ground of denial becomes mute. If you can, send the whole and completed application (AOS) with this request with a cover letter explaining that you are indigent and don't have enough income and are having a joint sponsor for the form I-864.
Good luck...