Well, to me saying "ils sont là!" when there is a crowd of 200 girls and one boy is still weird :) I would probably say "elles sont là!" despite it not being grammatical.
As for the genders of nouns, my native English French speaking friends are usually in pain.
I feel the same, and I think it's fair to switch to feminine in these cases. My point was just that it sounds a bit less weird than saying "elles sont là" when there is 200 girls and a boy.
As for the genders of nouns, my native English French speaking friends are usually in pain.