your scenario is unfortunately not well explained and creates confusion. it is unclear why you talk about components when you buy the material .
And as well the term "substitution" is not relly explained.
Whatever alternative unit you maintain, the stock is managed in the base unit and finally reduced in the base unit. And issuing parts of a good managed in EA is technically possible, SAP allows to enter 3 decimals, even this unit naturally does not have decimals.