Capture is a FireWire based software, so it can only record physically connected/cascaded inputs, which means that you need to cascade a second SL-AI mixer (any of the three) via FireWire to increase your channel count. Vice versa RM-AI mixers can only be cascaded with RM/L-AI mixers, which is done via AVB. There will be quite some second hand SL-AI mixers available soon, so economically this might be worth to consider. You would not need to buy any of the network option cards.
Another solution would be Dante recording, which simply means making any number of input channels available for recording in a network. You can't use Capture for network recording, but any DAW is suitable. A Dante solution costs some money, but many people are happy with it. There is a Dante part in the forums, so have a look.
Cascaded and networked recording is limited to 48 kHz, if this information is of any importance.