Is it possible to make Google Assistant to use a custom application (e.g WhatsApp) instead of SMS when I say "Tell Steve I'll see you in 30 minutes"?
Google Assistant recognizes commands for WhatsApp, such as “OK Google, send a Whatsapp message to [contact].” It will then prompt you for a message to send. You can't set a default texting app because Google recognizes "send a text/message to [contact]" as a command to send an SMS.
With an app like IFTTT you may be able to set a custom voice command to send a message if you do not like the default one from Google. However, as of now, I have not found a custom applet for WhatsApp messaging, but if you are looking for this functionality for other apps as well, such as Telegram, there are applets on IFFTT that will help you.