I thought the nominative plural of soused was sousedé???
Both are correct. Sousedé is more formal, sousedi more colloquial. I'd use the first one in writing.
I thought that "naši" was used only for masculine animate plural, and that nouns which end on "o" are neutral, no?
In this case the word "nasi" is connected with the word "sousede/sousedi", not with the word "Slovensko or Nemecko".