Your question was too long and so not all came through. If I can guess as to what you wanted to know, I would consider the live oak to be the good long term spreading shade tree option, although southern magnolia is also very nice but your heat zone is a bit too much for it. For the 15 gallon plant Roebelenii palm is an attractive specimen plant as is sago palm. Cardboard palm is okay for the area but not as showy. Ligustrum makes a nice bush or small tree also. I think if I were you I'd let my own aesthetic preferences guide me on the 15 gallon decision.
If you have additional questions please feel free to visit the home depot web site to ask more. If you'll keep them specific by dividing them into single questions we can be of more assistance.
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