For a short time you may get away with a 40 breeder depending on how the tank is set up along with the temperament of the male. They are extremely aggressive and it's common for the male to kill the female if he's ready to spawn and she's not.
They will start to spawn at a early age with the female around 1.5-2" and I generally have new eggs around every 6 weeks on average. They are extremely hard on tank mates during this which is common for central americans, I've had them kill larger festae guarding fry. So if dithers are used to distract the male make sure they can handle the punishment or at least are fast!
As the fish matures and ages this behavior only gets worse and the tank should be upgraded to a minimum of 48" of least 55g, although 75g would be better simply do water quality and better décor options. The largest salvini I've owned reached a little over 9" for the male and his mate was nearly 7". This pair was bonded pretty tight but even then it was common for the female to have tail fin damage and some bite marks occasionally. They would not allow for any tankmates to live for more than a few days so they were alone for a couple of years until rehomed them.
