Why do I have to use Erwachsene and not the german plural for adults, Erwachsenen, once I am referring to "adults" and not "adult". These are "adultS" =\
no article -> strong inflection
The reason for this is that 'der Erwachsene' is a nominalised adjective. These -although being nouns- get inflected exactly like adjectives.
"Das sind" should literally translate as "that are". Does "das" also mean "those", or there is another reason it is used here? Is "das sind" the only correct way to say "Those are"?
"When it is used with the verb sein, the form das can refer to singular and plural nouns of all three genders"
Yes, that's the only correct way to say "those are".