1. Oak is toxic, to a certain level, yes. Bull mesquite, no. Go nuts.
2. Got local quarries? Streams? Plenty of those 1/2 to 1 ton boulders will decorate nicely. Otherwise, river rocks.
3. Pea gravel is great. 1/4" and 1/8" river rocks are perfect.
Thanks. Lots of mesquite so that is good. No quarries anywhere near this place nor any streams of any type. Lots of beachfront close by!