Well the alternate outcome isnt generated from probability class, that one is used to generate one adoptable from several possible adoptable species(ie. when your two adoptables can produce offspring of multiple species).
The actual alternate status generation code is located at class_adoptable.php, in this method:
PHP Code:
public function getAltStatus(){
if($this->alternates == "enabled" and $this->altoutlevel == 0){
$rand = mt_rand(1, $this->altchance);
if($rand == 1) return "yes";
}
return "no";
}
As you see, this code generates alt status and it should work properly. if your altchance field is 100, it gives 1/100 chance to have alternate outcome. So theoretically, it should work if you adopt your pet through adoption center. But for breeding, I am not 100% sure so you need to test it out.