I want to use the I2C slave provided by pigpio with a Raspberry Model B. pigpio documentation says GPIO 18 and 19 are used for that. Unfortunately there is no GPIO 19 on an RPi1.
Is it possible to use some other GPIOs?
The answer is no.
The SPI/BSC Slave peripheral is only available on fixed GPIO. In the case of I2C this is GPIO18 for SDA and GPIO19 for SCL.
See pages 102 and 160 of BCM2835 ARM Peripherals.