Is it permissible for a girl to stipulate money other than the dowry, such as 80 or 100 grams of gold, in addition to the dowry? And is it sufficient for her guardian to inform the suitor of this, or must witnesses be present?

It is permissible for a woman to stipulate something in excess of the known dower, such as a specific amount of gold, and this would be included in the dower and subject to its rulings. Allah Almighty says: "And there is no blame upon you for what you agree upon beyond the prescribed obligation." And it is not a condition for this stipulation to be witnessed, but witnessing such matters is more definitive in preventing disputes.