If you want to use the main mixer to mix all tracks you shouldn't make a 14:2 at the top of the rack as everything will get routed to it and thus doesn't get individual tracks on the Reason main mixer. You can always reroute by creating mix channels and connecting the devices which are connected to the 14:2 to it. Each device needs it's own mix channel to get connected to the main mixer directly.
You can indeed use 14:2 mixers as subgroups for drums or other groups of instruments. But each mixer will end up on one channel on the main mixer.
Check out Dave909