Probably asked before... but:

Should the Chipjack Expert Driver be purchased for each slot in a multi-slot chipjack or it's applicable to every slots?

Thank you

Per slot.

Mind you, you can attack a jukebox to that slot, and get the benefit, but then you have to switch every time.

You don't need to buy Expert Driver for every slot in a Multichipjack if you don't need the extra dice in some of the chips. For example you could have EDs on the first two jacks and you might have no ED in the third slot because you just have a Lingsoft(5) in there, and you're pretty sure 5 dice will be enough for you to understand what people are saying in that language. You can always move the chip to a jack with EDs if the conversation starts getting technical.

As JesterX says, you can just have one chipjack and put good EDs on it then plug in a jukebox. If you control the jukebox using a separate datajack connection then you can activate skillsofts as a free action (and you're usually only going to be using one skill at a time anyway).

Also check out Cannon Companion p60. There are various options that reduce the effective load on your system, etc. but what is relevant here is:

That said, there's nothing quite as nice as having three CEDs, two in your firearms of choice and one in something else, so you have an automatically refreshing pool for each shot, plus your combat pool happily saved for dodging.