Ball Pythons dont require a cool down for breeding. Some people like to, others dont.
1500g is some people's base line for breeding. There is no magic number, they will breed when ready, which is usually their 2nd winter.
If you really want to cool them then there is nothing wrong with cooling all you plan to breed this season. Even if they havent hit the 1500g mark.
Good luck to you, hope you have a good season![]()