Can different brands of antifreeze be mixed.

Different brands yes I always just get the universal coolant.

But I would never do that. Some coolants can react differently when mixed so you must be sure of the grade/type you are mixing.

I have never had that issue its impossible to know what brand is in the cooling system especially when you get a used car. Even if you change the coolant there is still coolant left in. The system. The important thing is that match the type of coolant like dexcool etc. Also I would stay away from coolants with phosphates in them.