Certainly you may use just RO, but in all likelihood you will end up with a trace of TDS at the output. The problem is, there's no way of telling what that 1 or 2 TDS is comprised of. Probably harmless to you & I, but over the long term it will affect what's going into your aquarium. Probably not a big deal, but on the other hand, having the DI stage there is not going to slow down your production that much if at all and the DI media should last a long time if the membrane output is at 1 or 2. Adding the DI stage shouldn't break the bank either. You can still tee off a line after the RO stage to run to your fridge or a drinking faucet, whatever.
You're on the right track to get a handheld TDS meter to test your source water before buying a system. Another good idea is to get a pressure guage, should be less than $20 and not that hard to find. Most hardware stores and certainly irrigation system suppliers will have guages made specifically for measuring water pressure. I picked up two at Princess Auto. You want at least 60 psi for efficient performance. Water pressure, probably more than any other factor will have the greatest impact on production of good as well as waste water. There's more than likely a PRV (Pressure Regulating Valve) in your home plumbing just downstream of the water meter. If you can locate that, chances are it's adjustable. I've tweaked mine to provide just over 80 psi. I think most RO membrane performance numbers are based on 70 psi as well as a certain temperature which I don't recall at the moment.
Had a quick look at the report from the City you linked to and didn't see if they used Chlorine or Chloramine to treat the drinking water. You should confirm which since this will determine whether you need two carbon stages (Chloramine) or just one (Chlorine). I suspect a fellow Calgarian can jump in here & provide that info.
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