Your 16R has 6 mono flex/aux outputs, so if you want to use stereo mixes, there three available (outputs 1+2, 3+4, 5+6). You can also use the main mix and connect your headphones to the headphone socket on the front panel, so you can have four stereo mixes.
If you want to use mono mixes (6 auxes), you need a cable with a bridged connector on one end (mixer) and two separate connectors on the other end.
Mono is no fun, therefore you should try to get "more stereo". The 19" rack headphone amps have aux inputs, which you can use for this purpose. Send the main stereo mix of your 16R as signal to all 6 headhone amps, then add the individual aux signal from the 16R. This allows a basic stereo mix for all with "more me" (instrument, voice) added as mono signal. You will need 6 bridged cables to feed the aux inputs of the headphone amplifier, because Aux Ins are stereo.
