Verlan and slang change very very often. Especially if you talk to young people (can change every 2 or 3 years for instance), it is also sometimes very region-specific. (Paris, some suburbs, etc)
So you always have to learn the new word to say that or this, because the old one is out of date, and you may look ridiculous by using it.
That's the problem.
For instance, "Zarbi" to say "bizarre", is very outdated, it was used maybe 20 years ago (slang and especially verlan, change a lot according to your age, you may use a word, that will seem weird to another age group), but "meuf" is widely accepted as a slang synonym for "girl", a bit like "nana" and the English "chick".
My personal advise would be to learn these words, if you need to talk with people who use them, but not to use them out of context, with someone who is not as young as you, or with your boss, or with someone, even young, but who never use verlan...
Personnally, I like foreigners to try to be a bit formal, and find it a bit ridiculous when they try to hard to say a lot of verlan, but it's personal preference. Not all the young people use verlan. Some can make you look a little "racaille".
- "Tu" depends on your age, and the age of the person you are talking to. Generally, you need to belong to the same generation.
It also depends on the context (professional, friendly, etc...). Some people may become very angry if you say "tu" to them (it happens).
If you're not sure, ask the person his/her preference. A lot of people have personal preferences.