We respond to Rossano?

"Hello I have the black molly fry about a month and a guppy fry newborn, I can put them in the usual bath together?"

Hello Rossano, the difference in size should be such that there should not be problems.
If you had super-powered the first born, then you may realize that the last alone could enter into the mouths of the first ... and this is NOT good!
However, we speak of a separate tank main aquarium, because otherwise very few in the community and especially the first born of the last fry, could find shelter from adults, especially some species of characins, if you have any.
The only possibility is richly planted aquarium, with small-leaved plants, including the fry usalmente find shelter. However, there is often a security and the survival rate is only 5-10% ... so sooner or later at night or day, someone is distracted prey.

Also if you want to grow a good number of small fish, to introduce them in our aquarium, as gifts or to sell them, then a separate small aquarium is the best thing, especially because it allows us to feed the little ones in a targeted way, seeing them grow up close, change water frequently with minimal effort and possibly treat our little ones if they had the disease, without disturbing the large aquarium.

If anyone has suggestions to add ... the comments are below!