Can they be used interchangeably? For example: To work in ABC you first must gain wealth/breadth of knowledge about ABC's products
The difference between "wealth of knowledge" and “breadth of knowledge” is that they use different metaphors. Knowledge is an abstraction and can't be measured, so we speak of it using metaphors for things that can be measured, like personal wealth and physical distance (in two dimensions, whence breadth).
One metaphor theme is Experience is Money (put in his 2 cents' worth, high-priced education, not worth the effort). One can also have a wealth of experience, a wealth of ideas, or a wealth of opportunities; wealth is on its way on the same path as lot and may soon become another intensive quantifier. Here it's Experience that is metaphorized, and the possession of the Experience is possession of the Money, things the Experience (including knowledge, natch) can be used for are things the Money could buy metaphorically, etc. Money is also used to metaphorize Time in English.
The other is Knowledge is a Field (cultivate your skills, which field does he work in, as ye sow so shall ye reap). This is a bit more agricultural, with overtones of volitional labor, input and output, and plant themes. The bigger the field, the more the knowledge output.
Neither of them is really knowledge, of course. But they'll do to talk about it.
I don't see them as interchangeable. What about depth of knowledge. You could exhibit great breath of knowledge knowing one or two minor facts about each product. A wealth of knowledge seems to imply both breadth and depth of knowledge.