So you've got at least 2 new layers of plasterboard between the suspended ceiling and the
existing ceiling? In which case there's no need to worry about sound leakage through the
light fittings as the plasterboard will be doing the bulk of the soundproofing. I wouldn't
even worry too much about packing the void with anything provided you are using acoustic
tiles in the suspended ceiling. The function of the ceiling tiles is to control the
reverberation time in the room - not to provide any soundproofing.
In my last
studio we just mounted halogen downlighters in the suspended ceiling which worked fine.
JRP Music - Audio Mastering and Restoration.