Frodo an old work box in the tile will not work. It will cause the ceiling tile to sag and will not even be secure enough to mount a fixture. There are bars for evey kind of recessed light and ceiling box. These allow for proper installation. Voltron, here in Mass (not sure if its NEC too) the ceiling holds the fixtures however if they contain a ballast of some type they must be chained to the building structure. This way they remain hanging if the ceiling comes down.
