Our situations are slightly different as I had to fill out an I-864 (for a CR1 visa for my husband) and our embassies are different (in the end, it will be up to the CO), but here is what happened to my case:
When I first submitted the I-864, I was earning well below the poverty line. My I-864 reflected that salary even though I knew I would be getting a raise before the interview (you can only include your current earnings, not hypothetical/potential salary). I included a co-sponsor.
I received a raise (putting me several thousand dollars over the poverty requirement) exactly one month before my husband's interview. I was/am still at the same company, but they doubled my salary because I was moving back to the US and would continue working for them there. At the interview, I submitted a new affidavit of support (I-864) and asked to cancel the joint sponsor. I included a letter of employment, my one pay stub that included the new rate (I included older pay stubs, but only the most recent showed the change), and 6 months of bank statements showing regular deposits. The CO accepted it and approved my husband's visa. The embassy sent back the canceled co-sponsor's form/supporting evidence with my husband's passport/visa.
Essentially, to answer your questions, you can only include what your current yearly salary is at your current job. If there is an update to it by the time of the interview, you can have your fiancee submit a new form on your behalf. If you currently don't earn above the poverty line, you will have to include a co-sponsor (if only as a stand in until the interview; but the potential co-sponsor should know that it's up to the CO's/embassy's discretion...they might cancel the joint sponsor, they might not if they don't trust your work stability or whatever).
If you have sufficient proof that you are earning above the poverty line right now (employment letter, several pay stubs), then I think you technically don't need a co-sponsor, but if you just started your job, then it won't hurt to include one in the initial submission (to prevent possible delays) and have your fiancee ask to cancel the co-sponsor at the interview if possible.
