Thanks for the nice words. It sounds like we will not have to give you our usual tutorial on using the database. We are first going to give you some general cautions and tips and then answer your 4 questions, as best we can.
Have you been noticing the Light Requirements in the plants you have been choosing? It sounds to us like you will have full sun, which we consider to be 6 hours or more of sun a day. Part shade (2 to 6 hours a day) and shade (2 hours or less) are possible around structures or walls. Planting something that needs lots of sun in the shade could mean few blooms and/or something needing shade but growing in the sun can cause shriveling up and dying of plants. Going to all the work and expense of planting the wrong plant in the wrong place is a waste of resources.
Maybe we should have said this first, but don't plant anything now! The end of August in a record-setting heat wave and drought will only lead to grief and dead plants. You mentioned lighter watering of trees outside the main perimeter of your garden. Any new plant needs a good water supply in the first few months of its life, especially woody plants. A newly planted tree should have a hose pushed down into the soft soil around that tree and allowed to dribble slowly until water comes to the surface. This should happen at least twice a week, and sprinkling is not effective. This also brings us to the subject of drainage; you mentioned that the caliche is not solid so drainage should not be a problem. Actually, the type of soil you have also determines the drainability of that soil. The native soils in Central Texas are predominantly alkaline clays. Clay has lots of nutrients in it, but its very small particles swell when water is applied, basically making the water and nutrients in the soil inaccessible to the new little rootlets.
So, your Question 1: We suggest that the largest tree sustainable in your "raised bed" situation still can't be very big. Remember that the roots of a tree may extend 2 to 3 times as far out as the shadeline or dripline of the tree. We would suggest some smaller trees or shrubs that can be trained as trees, such as Ilex vomitoria (Yaupon), Cercis canadensis var. texensis (Texas redbud), Chilopsis linearis (Desert willow) and Sapindus saponaria var. drummondii (Western soapberry).
Question 2: Larger trees for farther back (but still within range of deep watering): Juniperus virginiana (Eastern red cedar), Platanus occidentalis (American sycamore), Quercus macrocarpa (Bur oak).
Question 3: We have no experience with gardening with any of the soil amendments you mentioned, and investigated them for ourselves. We don't want to prejudice you against any recommendations you have received from professionals, but just want you to know our opinion.
Chocolate loam-Basic soil, used for fill material, compacts fairly easily. This sounds like what used to be called "topsoil" and could have been anything from anywhere, and not necessarily including signicant nutrients. Since we have already mentioned you need to provide for good drainage in your clay soil, we don't feel that easily compacted soil is particularly desirable.
Dillo Dirt - recycled sewage product from City of Austin. Excuse us: Eeugghh! No doubt this has been allowed to cure for some time, and certainly must pass some sanitation tests, but we are not crazy about it. Sewage can contain damaging metals or other materials that don't "cure" out.
Sand - this would apparently be masonry sand, which might help with drainage if properly mixed with dirt, but water drains out of it very quickly.
Obviously, you are going to need to import some dirt for your "raised bed," but we would like to see it mixed with generous amounts of compost, followed by mulching with a good quality hardwood mulch. This would protect the roots of your plants from heat and cold and, as it decomposes, adds more organic material to the soil.
Question No. 4: Suggestions for plants. This is something we hope everyone in Central Texas heeds - the dreadful heat and drought, along with the unexpected cold snaps last winter, are probably not going to get much better. We are in the midst of almost catastrophic drought conditions, and any plant not suited by centuries of experience to these conditions does not belong in your garden. You must be prepared, although we hope not, to face years of low rainfall, water shortages, and watering restrictions. Gardeners should select plants native to their area, drought tolerant and pest resistant, in order that the considerable investment in resources will not be wasted.
