1. I have always heard from the coolant manufacturers and a coolant book I have, that ethylene and propylene glycols should never be mixed.
2. The best way to change coolants is to drain all the old coolant, and then flush the system with plain old water and run the car for a few minutes. Then drain all the water and fill with distilled water. Then drain all the distilled water and fill with your mixture of coolant/distilled water so that you obtain what ever coolant/distilled water ratio you are looking for.
3.Most forms of propylene glycol are more viscous than ethylene glycol, but the exact type of proylene glycol determines the actual viscosity in your engine. It probably won't make any difference when mixed with water.
4. A mixture of 50/50% propylene glyco and water and 50/50%ethylene glycol in water will have almost identical heat transfer characteristics. As you add more propylene glycol to the mix, the heat transfer will not be as good as ethylene glycol. It's true, but I don't know why it is true.
Are you guys SURE that mixing ethylene and propylene coolants is safe? I have always heard this is a very bad idea, though I have no idea what chemistry is involved.