Starting backwards like usual: No you do not need a seperate res, just watch you water levels close, the more it fluctuates the more the ppm fluctuates, the more stress for the plants. I run 4 plants in 6 gallons, in the biggest growth periods i can see 2-3 gallons gone in 1 day, so you must check every day, if not every 12 hours.
Second, the water temp issue, this has been my project this month so i am right on top of this one, wouldn't believe how bad 85 res temps will trash every root. Lets start with figuring out WHY we are getting the water so hot, i made several changes to my room between rounds and developed bad heat issues in my tubs, took me like 2 or 3 weeks to figure out it was the fact that so much of the light was being directed at 2 of the tubs it had no choice but to heat the water, my water temp shot up because i turned the light 90 degrees. Color of res and lid can obviously cause high temps in the water due to heat exchange. Black and white poly can help this, put the white side out and glue (i use spray adhesive) the black side to the res and lid, you have now removed all light from the res and made it exchange less heat, i would start with this (i don't know if you have plants in or are just testing at this point) but i would buy one of those cheapo res/fishtank thermometers and watch the temp as often as you can, but mainly as the light first comes on and then right before it goes out to see what your difference was across the day. I would not run a big light (600 1000 watt) for more than 12 hours a day, if it is your only light look into a 12 on 6 off 1 on 5 off, light schedule to prevent cooking your water too much, this will still prevent flowering. Just my thoughts but they are fresh in my head.
O and far as ice goes, if you are fighting fluctuations, its wayy too much work to try to keep in range with ice bottles, i was putting in 2 1 liters and 1 12 oz bottle everyday (changing them 1 1 liter at light on, 1 before i went to bed (about 6 hours later) and the 12 oz when the lights go out. Still was not enough to counteract the heat, and my deep freeze was fighting to have them froze back up in time. I could see throwing 1 1 liter to keep things in line, once a day, but i would just try to get temps stable and not worry about the ice. I did not use ice last time had horrible light leaking into the res (and roots) and was able to still average out around 3 ozs a plant dried.
I disagree with the turning the air pump off, if you are getting that much heat from your air pump, put in less stones. I use a water pump that is fed by a heat creating commercial air pump, both on 24/7 creating heat, no issues with heat from them.
Your air temp and water temp WILL be different, i would get a set up going without any plants and get a thermometer in there and see how things go.
AS far as the chiller, you will need 2 or a seperate res to use it for both of them, possibly plumbing them together would work, but now you have 1 system instead of 2 so every body has to be at the same stage and harvested (flushed at least) at the same time. You are at a good point to ask questions, but i would build it out at this point and watch temps then figure out what will work out best for your set up.
I aim for water temps to be 68-72 never getting above 75. If you ever reach in and the water is warm to the touch (around 80 this happens) then you have issues.
If you want to stay away from getting another res maybe the best way to do water changes is to get a 3rd matching res, transfer your lid to the 3rd one while you change water in 1 or 2 then move it back. I actually just cut an access door in my lid throw a water pump in there and pump out all the water, pull the pump and refill. I get like 99% of the water this way.
