Cytometric Bead Array kits are very sensitive. You can check one from BD Bioscience. You can customize a kit and measure simultaneously multiple cytokines/chemokines in small volume samples. I was very satisfied with the sensitivity of the beads in my experiments.
well, Cytometric bead array is more preferable method for doing multiple cytokines simultaneously. However, if you can't do this, i may recommend you to do ELISA in 50 - 75 micog of protein in plasma. By the way, which pathogenic condition, you are working on? you can check the saturation point of each cytokine by using their standards. After this, u can adjust your protein in the ELISA wells.