There were a few points brought up that I'll address.
In regards to over-sizing a system: With most water cooled air conditioning systems, over-sizing is never an issue. Your air handlers run on thermostats in the room, and never bring in outside air. So, when the temperature drops below, say 75F, it shuts off. When it shuts off, there is no heat transferred back into the water/glycol that is constantly running through the system. That is, when an air handler shuts off, there is still fluid flow through it. This will also go back to the temperature of the reservoir, which controls the chillers. I've seen some systems that run a quite simple design of staggered temperature chiller operation. That is, one chiller turns on at 44F, one at 45F, one at 46F, etc. The same time of things is common to see in typical A/C condensers with multiple compressors in them. That concept is something any system will have and it is designed to only use what is necessary.
I thought the plan was each room had it's own air handler off the main chiller coolant feed so CO2 should not exchange.
Each room will likely have multiple fan coil units which act as your air handler in the room. Some of these will be set for dehumidification, so this is correct.
In regards to cooling towers/dry coolers: It works better in dryer climates, so it depends on where in Canada. But, for reference, an 10 ton cooling tower runs maybe $2,000. A 10 ton water chiller from Hydro Innovations runs $8k retail. A 10 ton Trane condenser will run about $5-7k depending on the model. A 10 ton dry cooler(likely just the coil from a 10 ton condenser), will run maybe $4-5k?
Assuming 45F is a set point of your reservoir:
The dry cooler will use ZERO electricity, but only works when the temperature outside is below 45F.
The cooling tower works when ever the WET BULB(dew point) temperature is below 45F(ambient temp is above this point), but uses 5% the power of a chiller of the same size.
The water chiller will work down to about 0F, even with low pressure kits installed. Most normal A/C Condensing units stop running at around 30 of 40F.
So, in order to do it right, you need to have condensing unit(the water chiller) AND dry coolers. Cooling towers are optional in my opinion, but should be used if efficiency is the name of the game.