How are abilities determined for priority (e.g. if Tyranitar and Ninetales are sent out at the same time, does it become a Sandstorm or Sunny)?
They're decided by speed - the fastest inducer's weather goes up first, then the slowest cancels it. Being slower is therefore better purely to set weather, making Ninetales a bad lead against another weather team. Iron Ball can be used to slow your weather lead down if need be, however, as it impacts the total speed of your pokemon, and some weather mons use -Spe natures and less Spe IVs to be sure of getting their weather up (mostly Hippowdon).