If you use "e" instead of "este" in a sentence, is it preferable to be consistent and use "e" throughout?
You can use them interchangeably. "e" is just the informal,shorter version of "este" .
is the more informal "e" only used for singular or can it be for plural as well?
In the same escenary, I think it is best to use one of the two, be it the word "e" or "este". but do not mix, as for aesthetic as for consistem.