plasmonic resonances occur for highly conductive materials (gold, palladium,...) while resonant Mie scattering is mostly linked to particle transparency (usually dielectric materials in the optical domain): to observe whispering gallery modes light absorption must be as weak as possible...
Regarding your sub question... hard to say like that but, globally, in you add absorbing particles....
I am not so sure about my answer but i think depending on the nature of your surrounding medium adding gold nanoparticles may lead to more scattering from your inhomogeneous medium because the gold nano particle will enhance the field in the surroundings of it which may lead to more scattering. Hope this may you....