Q: Putting iridium spark plugs in '96 Cellica

Sure go ahead. Just realize that the only reason for using iridium plugs is because iridium is a precious metal that lasts a really long time. Inside the engine, there are lots of electrical firing of the plugs, and the explosion of gasoline gets super hot and very caustic inside. Iridium doesn’t wear out as much under these conditions, so these spark plugs will last longer. That’s all there is. You are not going to get better gas mileage. You’re not going to get better power.