Why an UNO? Why can't you keep using the solution that already works?
If you need something smaller than a Mega, consider the Micro. It has hardware serial independent of the USB serial, so you can decode serial data and send it to the PC without any messing around with other hardware or softwareserial. If you need true serial-to-serial, then a Teensy is even smaller and has good hardware serial for 3 ports, plus USB serial too.