djtypykuhl said:mollies and guppies are both of the genus pocilia, so follow ocminpin's post for a good guide line.
my mollies also started "going after" each other at about 10weeks old.
I just figured they would get pregnant as soon as they got to the LFS anyway so why worry about separating sexes. (assuming the LFS has males of their own... OR new owners will have a male)