2

The primary meaning of rud is a physical thing, whereas cursaí is the immaterial "matters" or "affairs". They only overlap in phrases like "things are bad", which don't refer to physical "things", but having said that, rud is used pretty much wherever you would use "thing" in English. For example, you would say an rud a rinne sé for "the thing that he did", even though you could be describing an action, rather than a physical thing. (Of course, an rud a rinne sé also means "the thing that he made", where it is a physical thing).

So tá rudaí go dona isn't wrong, per se, but I think Ta cursaí go dona is a better phrase to use, because it emphasizes to the learner that you are not referring to specific items being bad.

ní/nithe doesn't seem to be used as much (at least in the Irish that I've learned), probably because of the overlap with both the negative participle and the verb nigh, though it is still used, and, even if you don't use it yourself, you need to know it so that you can recognize and understand it in other peoples Irish.