I'd like to add a few tools from Kali to a debian wheezy. Tools like slowhttptest, p0f hping3 and a lot others.
Some of these tools cannot be found in the debian repos. Is it "wise" to add the kali Repos and install these tools?
I do not want to install all tools, just some specific ones.
Is there a chance it will break the system?
This sounds like a post for Debian forum! However, instead of adding Kali repo's you should be able to download most tools from git/svn/official-website and install them one at a time. Adding Kali repo means you end up getting more than you bargained for which most possibly will break something. There's some dependency differences between Wheezy and Kali.