Are the packs 60 trays or 60 cells?
60, 6 cell inserts.
Will these 6 cell trays fit in the 5x5 tray with humidity dome?
They won't fit in the 5x5 trays. They are designed to fit within the 1020 trays.
Will 12 of these 6-cell inserts fit into the 1010 seed starting flats?
You can fit 6 of the 6 cell trays in a 1010. However they won't sit completely flat. They are better designed for 1020 trays.