Can using Little Snitch make my mac Less Secure than its "out of the box" settings? Short Question
Im thinking about configuring little snitch to allow all processes incoming and outgoing network traffic. 

By allowing "any process” incoming / outgoing rights will this make my Mac less secure than its default settings (default out of the box, eg. not using OSX’s in built firewall tool)

Longer Question with background

The reason i want to do the above is because i want to block incoming and outgoing network traffic for a few applications. I can do this no problem with Little Snitch, but if i turn Little Snitch on it will by default block all network traffic and require each connection to be manually approved. To get around this i would setup the "allow any process" incoming and outgoing traffic rights. 

This way all incoming / outgoing is allowed as a global rule, but certain apps are blocked as a specific rule. 

Would doing the above make my Mac less secure than its default settings (default out of the box, eg. not using OSX’s in built firewall tool)
Little Snitch can work alongside macOS firewall. If a connection is permitted by Little Snitch, but is still blocked in the macOS firewall, it'll still be blocked. Little Snitch doesn't bypass the macOS firewall, so Little Snitch can't lower the security when the macOS firewall is still enabled.