Removing Authorisation/Encryption will reduce overhead considerably.
But you will need a seperate WLAN for the 100 users who do need A/E.
ZD 3000 tech specs: CONCURRENT STATIONS • Up to 10,000
ZD 5000 tech specs: CONCURRENT STATIONS • Up to 20,000
again these are ideal world numbers. Alsways better to have some excess capacity.
You need to talk to someone who does this kind of setup or one of the Ruckus staff who frequent here as is sounds like you are intending to do something fairly major.