Copper, nickel, brass, all fine.
Using purpose coolant is advised, despite many saying otherwise. Simply go clear mayhems, if don't want the hassle of striping blocks when changing colours.
The only thing that I would avoid like the pest is unbranded God-knows which manufacturer radiators, usually the very cheap from the bay or rainforest. They may say copper, but I bet my wages and my left leg that they have aluminium on them, which would give you nothing but headaches, unless you know exactly what you want and go full aluminium. Nothing wrong with aluminium, but to keep things simple, either go left field and get all aluminium (not sure how is the availability of blocks) or go mainstream copper/nickel/brass.
Back to your question, no issue.
I would only recommend to clean the block to ensure no contaminants, that may be present, react with your loop.