Q1. 今天没有下雨 expresses the weather condition that it did not rain today. 今天不下雨了emphasizes the consequence that the rain has stopped. Q2. 俩 means two, is almost equal to 两个，just much more casual than 两个. so yes, it is used much more often in Spoken Chinese. e. g. 我们俩去看电影吧？ 这俩人真有趣； 这俩词意思一样； 你们俩不要再淘气了 ...etc.