The interrupt vector table for the 32u4 shows this for vectors 9 and 10.
9 $0010 Reserved Reserved
10 $0012 PCINT0 Pin Change Interrupt Request 0
It's possible that adding the PCINT0 would conflict with a software serial use of interrupt vector 10. I would think more basic issue with software serial and interrupt vectors should be present even before you add the pin change interrupt vector.
Can you link to the specific library you are using for the 2209?