I put two guppy males in with one of my gentlest boys... they were tail-less and dead within 10 hours.
I had one tiger-spot with vertical inch of tail with the girls and nobody ever nipped at him.
I started with two boys and four girl guppies. My two males were also killed quickly. Only the girls were left alone. The boy guppies were very pretty though, so that is probably why the male attacked them.
I heard guppys were not safe becuse their tails are like bettas (mistaken identity) I have a lot of ghost shrimp and 2 ADF in my 20 gallon sorority/communty tank I did have danios but had to move them because they are pigs lol
Guess it varies. I have a male fancy guppy in mine, bright red gorgeous tail. All 5 of my female betta's ignore him.