Ok on the multiple leds.
But I was thinking of Charlieplexing more in Arduino terms... as in that library.
But yes. You can turn on more than one led at a time.
But, that makes it difficult to calculate a resistor value for current limiting.
For any fixed resistor value, the led brightness will vary, depending on how many leds are on at a given time (in a common group).
It's probably better (at least would be more consistent/easier) to stick to 'one at a time'.
Or maybe PWM would help solve the multiple leds on current/brightness issue? XD