No, the 8 "unused" mix outs of your RM16AI work only when two RM mixers are cascaded over an AVB network and the second mixer is a RM32AI, which has the physical outputs for the mixes 9 to 16. Only RM/RML mixers can be cascaded/connected to one operating system. This is the same with the SL AI mixers and the other interfaces. They can be used only in their "family" of products. Of course you can add any mixer for more input channels, but this does not give you more mix outs. That's why I think a single RM32AI is the best solution for you.
But what about changing the way you use the 8 mix outputs? If you would create a stereo mix for all musicians and add an individual mono signal, you could serve 6 musicians. It goes like this:
#1+2 stereo mix for all musicians
#3 musician 1
#4 musician 2
#5 musician 3
#6 musician 4
#7 musician 5
#8 musician 6
Of course for this you need 6 headphone amps which allow to add mono mixes (usually via insert). If you own them, this is an easy and effective solution. If you would need to buy some, investing in a RM32AI instead might be the better alternative. For musicians true stereo anyway is more fun.