does reverse light plastic (grey w/ logo and red) on the trunk from a '95 Q fit a 90 Q?

absolutly. id did this swap on my 91 to get the chrome ring around the lights and trunk panel. keep in mind if you only use the finishing panel there wont be any chrome to make it look finished around the tail lights.

Is the trunk lid between the 1st and 2nd gen Q45 interchangeable?

The lid does not interchange between the G50 chassis and the FGY33 chassis.

To be clear, we're thinking of 1st Gen as 90-96, and second Gen as 97-01. I'm pretty sure 90-96 will all fit each other.

Sorry, I was thinking of 90-93 being first gen and 94-96 being second gen, I know 90-96 are the G50. I know they have the same body style but the minor differences to the rear, hood and interior always made me think it was a 1st and 2nd gen between the 90-96 years. But my question was answered, I wanted to know if the trunk lid on a 90-93 can interchange with a 94-96? I swapped a carpet from those years and it didn't quite match up, but I still made it work and I still have it in my car today. Thanks for answering my question Q451990, sorry for my confusing or incorrect post 3Q Jay.

all good. I would agree that the G50 kouki and G50 zenki lids can physically interchange...not sure about all the trim bits (deck lid finisher obvious, other things might have subtle difference (sound damping, etc)).