Re: plural noun after a preposition
Just to point this out: This has nothing to do with prepositions, as the thread's title suggests.
These are called "Distributive plurals" and there are many threads on this topic on just about any ESL forum you can find.
The only thing that is important is to make sure that there is not ambiguity that would make it hard to understand what the main point is.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.