MILAN, Italy — Simone Rizzo and Loris Messina, the young guns behind up-and-coming label Sunnei, came to fashion via marketing. Their catwalk debut yesterday was a catchy, jolly and well orchestrated line up of ironic separates that looked like the perfect modular uniform for a party at the madhouse. Although not designers, and albeit clearly commercial in their aims, the two have vision, and a sense of humour, which makes a difference.