Guppy + Molly?

[Nuttawet]

Can guppy fish live with molly fish?

 

[Gotfins]

Not only can the live together, they can interbreed

 

[Colin_T]

They can't hybridise but they can live together if you have hard water (GH around 250ppm) and a pH above 7.0.

You also need a decent sized tank for mollies because the sailfin variety can grow to 5-6 inches long. Even the standard black molly can grow to 4 inches long and needs a tank that is 3 foot long or bigger.

 

[Gotfins]

They can't hybridise but they can live together if you have hard water (GH around 250ppm) and a pH above 7.0.

You also need a decent sized tank for mollies because the sailfin variety can grow to 5-6 inches long. Even the standard black molly can grow to 4 inches long and needs a tank that is 3 foot long or bigger.

They can and do cross breed.

 

[emeraldking]

They can and do cross breed.

You're right that they can crossbreed. But there's a "but"...
If the tank contains both genders of both species in there, it's very unlikely that they will crossbreed. But if you only keep the opposite genders of each species in one tank, the possibility of a crossbreed is much higher. For if both genders are present of both species, the female will prefer a male of her own species as a mating partner. For it's the female that will decide wether she let's a male mate with her or not. No matter the heavily chasing of the males all day long.
But if you'd like to establish a hybrid of them, choose a female molly and a male guppy. In case, the size of each individual newborn fry is large, it's better to let a molly give birth than a guppy.