We don't know how long it takes for intermixed coolants to react and develop significant precipitation/sediment. Did you check the coolant when you purchased the car?

Have you had the oil changed at a shop where a technician may have topped up your coolant?

 

Just discovered I have no "spare" coolant for my current cars!!! Anyway trying some Coregaurd from Preston a 50/50 mix to see what happens to a friction band from a ZF6HP26.
The temperature is only about 43 C so that might make a difference in the outcome.

 

After soaking part of the friction band in coolant overnite absolutely no impact on it at all.

Whether that is meaningful or not I dunno.

However I do know that if coolant were to infiltrate my ZF6HP26 transmission I would flush it out and refill with Mercon and see what happens. Of course after doing repairs to the
cause of the leak.

 

Any update information Pristine97XK8Convertible ?

 

Just wanted to add that if you are concerned with the effect of coolant in your transmission, once you get your issues sorted out, drive the car for some time, swap out the fluid and send some to a lab to have it analyzed. Coolant does not contain anti wear additives and can potentially cause issues if it was a long standing problem. If it were me I would probably do a few flushes over the summer and have the first and last fluid change sent off to Blackstone or similar to see what amount of contaminants were left.

Oh and for the mixing of coolant. It is a bit confusing. If you are mixing old school green and Dexcool, you will get a gel. However, if you mix say dexcool with a universal extended life that is all over shelves these days, you shouldn't have any issue. Many are formulated to mix with any coolant.