Yes, you can use most analog pins as digital inputs. pulsein() waits for a pulse and returns the length of it in microseconds. It seems from your sketch that the fans give 2 pulses per revolution, but we don't know for what part of a revolution the pulse lasts. Have you any information on this?
For example if a pulse lasts one quarter of a revolution, and pulseIn() retuns a value of 5000us, we can calculate the rpm as 60 × 1000000 / ( 4 × 5000 ) = 3000