We evaluate HI surveys as a cosmological probe of small scale structure and attempt to determine the survey criteria necessary to test ultra-light axion (ULA) dark matter models. Standard cold dark matter (CDM) models predict a large population of low-mass galactic haloes, whereas ULA models demonstrate significant suppression in this small-scale regime, with halo mass cutoffs of 1012 Msun to 107 Msun corresponding to ULA masses of 10−24 eV to 10−20 eV, respectively. We conduct mock HI surveys as they would appear in CDM and a variety of ULA cosmologies to determine the detectability of these cutoffs in observational reconstructions of the HI mass function (HIMF) and corresponding halo mass function (HMF). We generate random, homogeneously populated mock universes with cosmological parameters adjusted to match CDM and ULA models. We observe the mock universes with hypothetical analogs of the mass-limited ALFALFA and WALLABY surveys. The resulting statistical previews of CDM and ULA universes allow us to gauge the capabilities of these surveys through the lens of ULA cosmology.