From my personal experience (death in my community tank) 3 type of fish died, 1 didn't get affected, few others survived as if nothing ever happened.
Still have 3 neons and 4 guppies and 1 angel in there alive and well(4-5 guppies died within 2 days and betta died in the second day, tetras kept falling from 10 to 3 in matter of 3 days)
Now its like a week and no deaths.
Guppies started dying before the betta showed symptoms.
I'm only guessing this but I think most stuff that will hurt a betta will hurt a guppy(long family relatives or something maybe, but less strong for sure).
Potassium Permanganate (PP)