Using slang

So I'm writing a Prohibtion-era period piece (I use this term loosely), and this is a thing that's been bugging me the whole way since the start. Should I use traditional Italian American slang like you would expect to find kibitzing with the mob? If so, when is it appropriate, and when should I resort to more conventional language?

I ask because there are situations where I feel it would have more of an impact to use more familiar American vernacular (for example, calling someone a "fucking asshole" instead of a goombah or a jabroni), but I don't want to be inconsistent in my use of slang for fear of breaking willing suspension of disbelief.

Personally, I love reading historical fiction where there's little touches like real slang thrown in. It adds to the atmosphere, and once it's not forced, it makes the whole world more believable, and shows there's been research put into it.

So I'd say stick entirely to the slang that was used back then if you can. If some of the words really don't suit the mood, and if you really really can't find anything else to use, only then go to modern day slang.

You know that thing where you have to alter the respectfulness of your speech depending on who you're speaking to? I'd keep that in mind as well. Local slang is often confusing to outsiders who have only had the opportunity to learn the basics, after all.