The Royalton, a downtown Miami hotel built in the 1920s, is back in the urban mix, but its tenants aren’t tourists. It’s now home to 100 low-income residents, 80 of whom were recently homeless until its reopening earlier this month. The unusual project is the product of a collaboration between a nonprofit agency and a for-profit developer of affordable housing. They raised $18.5 million by combining Wall Street investment, government grants and federal tax credits given to encourage renovation of historic buildings. The project’s nonprofit partner, Carrfour Supportive Housing, will provide intensive services, including in-house counseling and help with finding jobs, to ensure its formerly homeless tenants — all of whom have leases and pay rent — remain securely housed. Residents must have been clean and sober for six months prior to arriving and must submit to random drug testing.