General guidance may be... too general. One learner recently found a mistake in Wiktionary tables, caused by something wrong in templates. Better to check specific words, I guess (although of course true, some generalization is very useful).
If you haven't seen it yet, this website maybe has some small mistakes sometimes, but it generally is pretty awesome: http://mowicpopolsku.com/
"dziewczyna" and "chłopak" are the more 'adult' versions for "girl" and "boy".
Well, in (my) English I would say that a "sheet" of paper is something that maybe you get out of a pack of A4 or legal paper, whereas a "piece" might be a torn/cut off fragment of a "sheet". I think that such a piece would be "kawałek papieru".
EDIT: In some variants of English (e.g., USA), it is understood that "piece" can be my (UK) "sheet", and indeed, Duo accepts either.