is my 96 geo metro 1.3 an interference motor?

if you were to get any answers at all from the internet then i highly doubt they would mean anything close to what your mechanic means. the term of "interference" that he is talking about is the fact that the valves in the head run at very close tolerences to the crown (top) of the piston and that if the valve timing is not correct or in your case break a belt then some of the valves are left in the open position causing the piston crown to strike the valves, creating serious engine damage. If you are still unsure then i am fairly certain that in engeneering terms interferance is explained by an object that is slightly larger than the area it fits into creating what is known as interferance fit. in your engine the valves being exposed create a lager area than can fit when the piston comes to the top!

Yes, you will [at the least] bend the valves if you break the timing belt. Not so with the 3cylinder engines