Hi, you need to first find out why the coolant and transmission oil mixed, it’s not normal, I’m not very experienced with Xterra specifically but it’s still odd for the two fluids to mix.
You can always go with a transmission cooler for obvious reasons, it will definitely help in the summer, especially if you are going on more demanding drives. Installing a cooling system will extend your transmissions life. My truck has a trucool system for the transmission as I had overheating issues when climbing very large dunes in the summer, after installing it, I haven’t had any problems
Im sure the Xterra owners in Carnity will be able to share their inputs and experiences.