What constitutes "specialist coolant"... anything with corrosion inhibitors? I was under the impression that using automotive fluids (like anti-freeze) in a loop was harmful to plastic parts inside a pump. Im wondering if something like Feser Base would suffice. I have some copper radiators on-hand that Id like to use but mixing metals just sounds like it's asking for trouble.
It's around 10 dollars for pre-made kit I suggest getting it. It is to prevent corrosion among other things.
You can make your own yes. But your risking a lot if done wrong. Most common is straight water, no you should not do this. Anti-freeze I cannot say I have heard of tests but I would not want to be the first to try it.