Ask Mr. Smarty Plants

Question 1: If you intend having only native plants in your flower bed, you don't really need to do anything to your soil. Native plants are perfectly happy in native soil and often resent a lot of soil amendments, especially organic matter. This is sort of the opposite of most gardening practices, but that's just the way it is with Texas native plants - they're adapted perfectly to our poorer, more sere conditions and suffer from too much love.

Question 2: If you need to add soil, you should purchase topsoil from one of the many soil dealers/garden centers in the area and spread the soil no more than 1" deep around your plants in any given year. Adding too much soil will suffocate the roots of your trees and shrubs and they will need a year or so to grow new roots into the new soil. Also, it should not be worked into the old soil too much if there is a possibility that this will disturb the roots of your existing trees and shrubs.

Question 3: Generally, there aren't too many native plants that are going to bloom in Austin in December, January and February; and after the first freeze, many perennials die back to the ground to emerge again in March or April. However, there are a few exceptions:

Cenizo (Leucophyllum frutescens) is evergreen and has been known to bloom every month of the year.
Prairie verbena (Glandularia bipinnatifida var. bipinnatifida) is reported to bloom March through December, and I have seen them blooming in the wild in January and February as well.
Greenthread (Thelesperma filifolium var. filifolium) begins blooming in Austin as early as February and may continue through December.
Straggler daisy (Calyptocarpus vialis) is a low, evergreen groundcover with small yellow flowers that blooms all year round.

There are several native plants with evergreen foliage that bloom later in the year:
Texas yucca (Yucca rupicola) blooms April through June.
Mountain laurel (Sophora secundiflora blooms February through April.
Yaupon (Ilex vomitoria) blooms April and May.
Evergreen sumac (Rhus virens) blooms July and August.