Hi,
If I remember correctly, it is nr of seats and if its a jet or not rather then weight that decides if you need one or not. Above 19 seats its a requirement and/or if its a jet... TCAS I or II? I believe its becoming a requirement that all types required to have TCAS should have TCAS II from a certain date/year. This Im not sure of, but think I read something on this a few months ago....
/CP