Are INTx pins the only ones eligible for Software Serial? As opposed to PCINT (Atmel's Pin Change INTerrupt, INTx is arduino-compatible interupt)  In other words, does Software Serial use atmel's pin change or arduino falling interrupt?
SoftwareSerial uses PCINT.  AltSoftSerial uses INT0 or INT1, which has FALLING, RISING, CHANGE or LOW LEVEL options.  See Section 12.2, p. 71 of the Atmel ATmega328 spec, for example.