Additive -
https://www.napaonline.com/en/p/MCC1300
If using water, use distilled water.
Even though distilled water/additive cools the best,
green antifreeze keeps the rust away better than water with additive.
If converting from one type of antifreeze to another, cooling system must be thoroughly back-flush/flush out the old antifreeze before putting in a different type. Never mix antifreeze types. This is advice I've received from multiple professional mechanics including those who work on As and classic/antique cars.
Use green antifreeze, some of the more modern antifreeze types can eat away at solder.
If using white vinegar, do a final flush of baking soda/water to neutralize remnants of the vinegar that seeps into the metal. As mentioned above, the acidic content can continue to slowly over time eat away at tthe cooling system. An analogy is what happens to metal battery holders if not occasionally wiped down with baking soda/water.
Rust 911/Thermocure are not acidic, do not damage metal/rubber, only "dissolves" rust. It is important to 1st do a treatment of detergent (Dawn, etc) to flush out grease /oil. Thermocure/Rust 911 does not remove oil/grease, so any rust under the grease/oil is not affected/treated by the rust911/thermocure treatment.
Leave the rust 911/thermocure treatment in and drive the A every day for a week, then drain. Depending how much rust is in the cooling system, you may have to do more than one treatment.
If your radiator is very old and in poor shape, treatments may cause leaks from the removal of rust acting as a plug that was keeping it from leaking. This seems to happen more when using an aggressive acid treatment like white vinegar.
When done, install an upper hose coolant filter, to protect your clean radiator from further crud from getting into the radiator.