Breeding chance is easy and I have it on my site.
Make a new field in your adoptables table called bredchance and set it to whatever number you want.
This would go somewhere in your breeding script.
PHP Code:
$bredtemp = rand(0,100);
if($bredtemp <= $bredchance){
$article_content = "These two don't seem very interested in each other right now. Try again in a week.";
}
else{
run query to insert bred offspring