https://www.kali.org is a good distribution because it has allot of tools ready for use, or available on the repositories.

I use Kali Linux Light (with some extra tools installed afterwards) in a virtual machine, and it works for study/testing. Not so much for wifi related stuff.

USB is a good solution for "working" with Kali, it _could_ work with wifi, but not so much for study/testing.

And even if adding a new user is not difficult, I can't see Kali as a main system or in dual boot. It is after all a dangerous tool.

And there are other security centered distributions around, like https://www.backbox.org or https://labs.fedoraproject.org/security/; my advice would be to run an entire lab in virtual machines, attack and victim machines alike (try http://sourceforge.net/projects/metasploitable/ for example), and choose a distribution after trying more than one.