"I do not need expensive soap."
Translation:Nie potrzebuję drogiego mydła.
so there's no difference between whether you want the whole soap or just a little bit of it
As 'mydło' is uncountable in Polish ('kostka mydła' is the unit ='a bar of soap') - yes.
Though in everyday speech you can sometimes hear something like 'Kup dwa mydła' ('buy two soaps'), as people tend to make it shorter - and easier.
what about with other words, do you use a different case whether you want the whole thing or just some of it (like in Russian, you use one case if you want the whole thing, and another one if you want some of it)