Dwarf yaupon holly (Ilex vomitoria) is readily available, fairly inexpensive in 1 gal size, takes full Texas sun, reasonably drought/pest tolerant, tops out around 3 ft, and would remain a pretty dense windbreak year round. The main downside is lack of seasonal variation. It will be rock solid reliable, but could become a rather boring border in the quantity you're needing. It will also take 4+ years to approach 3 ft from a 1 gal start.

If not in a rush, you might experiment with autumn sage (salvia greggii). Though not as dense as the yaupon, it should be nearly evergreen in your zone. Height falls in the 2-4 ft range depending upon the cultivar. It grows quickly and is fairly easy to propagate from cuttings, so you could get all the plants you need within a season or two from a single parent. Red flowering is probably most common, but many others are available. Size and shape do seem to vary a bit by cultivar/color, so may need to experiment if you want to mix them up and get the look you are after. Weaknesses are low tolerance for wet feet and brittle branches. Brittleness is offset by rapid growth, but could be a problem if there are pets that like to check out the fence.

Cenizo, a.k.a. texas Sage, a.k.a. barometer bush is the best thing that ever happened to our fenceline, and if I had my way, I'd take out all the rest of our redbud and holly and boxwood hedging to replace it with that. The advantages of aesthetics are huge, because it is a gorgeous plant with a gorgeous flower, as well as how I never have to water it but every once and a while. The less I water it, the better it does. (Unlike those thirsty, annoying boxwoods!)

I'm also a huge fan of double-knock-out roses for security hedging. Kumquats and Calomondins, too, if you're southern enough for the citrus.

Rosemary?

I think Knockouts would be great and you could get them at a good price since so many places have them.

If I had to work with an iron fence, I would be tempted to use sweet Tea Olive. I love the leaves and the fragrance. Mine takes brutal west sun and heat and minimal water and keeps coming back. It is not tree like, rather is a shrub.

I wish I had a place to have a row of them

c

mexican oregano, shrubby germanders ( teucrium ructicans), Teucrium chamaedrys (upright germander)