Yes, Mosteiro is a good place to start the Central from Porto, avoiding the Porto suburbs. To start there, take the Porto subway (red B line) from downtown Porto to the Vilar do Pinheiro station (4 zone fare). When you get off the train, head north and take the first right. Follow that road, crossing the highway (N-13) and continue on the same road until it ends at a T intersection where you go left, putting you on the Portuguese Central Camino. You will immediately arrive at a village commercial area, with a nice cafe on the left side of the Camino route where you can also get a stamp.