Thanks for a nicely formatted post. This is gonna help many new players. Few things:
There is minor typo with "that" above and from my experience, when a swarm happens, there are always 4 pokemon. They're not always within the same grass patch but they're always within proximity. Idk if this is true for all swarms but if it is, then it is wrong to say 2-4.
Also smeargle swarm can happen in artisan cave, which is their most common location. It might be better to reword that and say something along the lines of "pokemon that appear in overworld as sprites" or something like that instead of saying they appear outside of their usual location. This is your guide after all and you can word it in any form you want.
Moreover, the English name of Undy is Unova. Thanks again for the guide, +1.
As for documenting swarms for that forums, I have poke caught in swarms so I know their locations and catch times but I dont know their start and expire times. Do you want me to skip those entries or try to add those without filling the times?