compatibility question

Would a aftermarket Catback exaust for a Civic EX fit on a Civic DX?

probably not directly. dx/lx cat is infront of the motor, ex is under the car. might need the exhaust extended a bit

Would a aftermarket Catback exaust for a Civic EX fit on a Civic DX?

To use an EX exhaust on a DX, you have to use the entire EX exhaust system from the exhaust manifold to the catalytic converter to the mid pipe and muffler.
all the hangers are in the same place, thanks to Honda's one-size fits-all construction.

If you run into problems most decent muffler shops can fab some pipe so that everything will fit. Most exhausts I have had done were custom front to back, all you have to do is pick a muffler and cat then off you go.