You don't need to know anything about the units. You can either reuse the standard Blizzard tooltip (change its looks and add to it) as I do in TipTop, or you can create a new tooltip object and use the appropriate functions to set the mouseover unit.
It may also help if you clarify what you mean by "tooltip addon". What will your addon actually do? Do you just want tooltips to look different? Do you want to add information to tooltips? Do you want to add extra tooltips? Something else completely?