  We present results from the characterization and optimization of six Skipper
CCDs for use in a prototype focal plane for the SOAR Integral Field
Spectrograph (SIFS). We tested eight Skipper CCDs and selected six for SIFS
based on performance results. The Skipper CCDs are 6k $\times$ 1k, 15 $\mu$m
pixels, thick, fully-depleted, $p$-channel devices that have been thinned to
$\sim 250 \mu$m, backside processed, and treated with an antireflective
coating. We optimize readout time to achieve $<4.3$ e$^-$ rms/pixel in a single
non-destructive readout and $0.5$ e$^-$ rms/pixel in $5 \%$ of the detector. We
demonstrate single-photon counting with $N_{\rm samp}$ = 400 ($\sigma_{\rm
0e^-} \sim$ 0.18 e$^-$ rms/pixel) for all 24 amplifiers (four amplifiers per
detector). We also perform conventional CCD characterization measurements such
as cosmetic defects ($ <0.45 \%$ ``bad" pixels), dark current ($\sim 2 \times
10^{-4}$ e$^-$/pixel/sec.), charge transfer inefficiency ($3.44 \times 10^{-7}$
on average), and charge diffusion (PSF $< 7.5 \mu$m). We report on
characterization and optimization measurements that are only enabled by
photon-counting. Such results include voltage optimization to achieve full-well
capacities $\sim 40,000-63,000$ e$^-$ while maintaining photon-counting
capabilities, clock induced charge optimization, non-linearity measurements at
low signals (few tens of electrons). Furthermore, we perform measurements of
the brighter-fatter effect and absolute quantum efficiency ($\gtrsim\, 80 \%$
between 450 nm and 980 nm; $\gtrsim\,90 \%$ between 600 nm and 900 nm) using
Skipper CCDs.
