LeafBlade12345
Songster
I have kept Bettas with male guppies that have very small tails. Male guppies with huge fancy tails attract too much attention and are too slow to be able to avoid attack of a male Betta.
You would never be able to keep a Betta with guppies that you are trying to breed. They would eat the fry as fast as the female has them.
I'm sure the bettas have different temperaments. Mine all attacked my guppies, these are hobby fish, so have very small tails, and killed one. I won't take the chance again, especially with my breeders, but perhaps your Bettas are different.
Regards,
Leaf