Q. Would Acacia longifolia do okay in the Las Vegas area?
A. San Marcos Growers reports cold hardiness down to 20F so it will take temperatures similar to Myers lemon, a fairly cold hardy citrus.
Click for A. longifolia at San Marcos Growers
It is possible it might have dieback during some extremely cold winters but it should re-sprout easily from its base once it has established for one season. Like any Acacia I worry a little bit about allergies from its pollen. It is a rather large plant usually grown as a shrub but I think you could train it into a small tree fairly easily. It should handle our soils with little problems just make sure that you amend the soil at the time of planting with compost.
Anyone with experience with this plant?