Should I connect my I2C devices to pins "23 PC1 (D 17)" (SDA) and "22 PC0 (D 16)" (SCL)?
Will the wire library work without any modification?
Yes, this works just fine. I'm using the '1284p in a project and I2C / Wire works fine as-is without any software modifications.
I can't definitely answer your second question. Anecdotally, I have used I2C devices in a mixed 3v3 / 5v project and I tied my pullups to the 3v3 line. The I2C devices functioned just fine. I didn't even realize the Wire library activated internal pullups on the '328. Your mileage may vary, it probably depends on how sensitive the I2C device is.