If your grandparents were living in Flower 5th Avenue in mid or late 1900, they should appear there in the notebooks carried by the Census takers that year. These notebooks are available at the NYPL at 42nd street.
They should be in those notebooks wherever they were living. I think that there is a sort of index to names in the 1900 census. They will be in the 1910, 1920 & 1930 censuses, too; 1940 will be released in a couple of years.
George A. Thompson
Author of A Documentary History of "The African Theatre", Northwestern Univ. Pr., 1998, but nothing much since then.