You can make several contact clouds, and you can control per cloud what action happens when you tap a contact.

So you could make a cloud for just your WhatsApp contacts, and set WhatsApp as the default “Bubble Action” for that cloud only:

If you don’t like the idea of multiple clouds of people, you can add a Bubble Folder inside your existing cloud of contacts, and move the WhatsApp contacts into this inner cloud.  You can change the bubble image for this folder to have a WhatsApp icon (from any free icon pack).

I show how to achieve this in the following short video: