I bet a range that wasn't native to elk could get wiped out by them pretty quick.
Historical Merriam elk range included almost all of west Texas, much of the panhandle and parts of the hill country before they were extirpated from Texas back in the late 1800's and early 1900's. Before the great plains became the farmland of the world, elk would range all across the flatlands grazing and then winter in the mountains. They are also native to areas of Kentucky, TN and Virginia as well.
Elk and mule deer may compete for the same food, but in every other state they seem to co-habitate very well without huge effects on each others numbers.
I'm glad to see elk coming back to TX - they are fun to hunt, awesome to hear bugling during the rut, and are massive animals that provide a huge amount of some of the best venison you can get.
A bull will keep an average of about 60k acres as his home territory and cows tend to stay around 38k acres.