Programming Help Needed: Mix and match product discount in same category

Hello,
I am looking for discount coding for version 1.9.
Where discount will work like this:

Discount will apply any product in a certain category.
If 1 product is added to the cart then there will be no discount. Say the full price is $25.
If any other product is  added to the cart will automatically discounted,  say $5.
So, if you have 3 products in the same category  then the price will be ($25+$20+$20).

I need this one very soon and I am willing to pay for the coding.
Thanks in advance.

I recently contributed a discount rule based on categories, but it's for version 2.4, so it's probably not going to help you.

As far as I can tell, there was no mechanism for adding discount rules as plug-ins in v1.90.

Well, I don't need a plug-ins. I can change one of the rules if you can provide me the code for 1.9.