I've traded and swapped chickens many times in a single, large outdoor flock for years and I've never had any issues from it. I think biosecurity only matters when people make it matter by having unnatural, sterile environments and sickly birds in ivory towers
I would recommend re-homing or eating all of the dark males, and keeping the one lavender roo. Lavender and black are the same gene, so you would probably get beautiful mixes in the future that way