Is wassup hussla? (also gangsta, killa etc.) appropriate to say to a friend or a colleague? I mean, I know that it's not any sort of an official phrase I'd say to my boss. What I mean to ask is whether this word may be taken offensively, like nigger in the US.
The word is hustler, and it can be very appropriate depending on the situation. At it's broadest, it means nothing more than "somebody who will put in the effort necessary to get things done". In the darker corners of the streets, it carries a different connotation ("one who will do anything to make a deal"), but the word lives a respectable life in the suburbs as well.
Killer may be an appropriate nickname as well -- it doesn't necessarily mean, well, a killer. It could mean nothing more than, say, a reliable closer in a sales department.
Gangster (or any street-spelling variation thereof) is probably not appropriate for the workplace -- even if your are working for, um, "legitimate businessmen" whose legitimate businesses seem to do very little actual business. They have their own preferred vocabulary, and it doesn't involve interesting any outside parties who ought to stay disinterested.
Everything can be taken as offensive. Some may confuse your "hussler" with the porn magazine, and thus take offense.
It can be... nah, I can't get myself to call "appropriate". "Inoffensive" it may be, but even though I'm generally used to very informal workplaces, I wouldn't call a co-worker "hustler". It doesn't just depend on the workplace, it depends much more on how friendly or maybe even close you are to/with your co-workers.
Or are you working in a pharmaceutical company? You know, with other "drug dealers"? ;-) Then it may... well, it would still be extremely informal.