Hey mark welcome to permies. You can train a rose into a tree shape pretty easily, you just prune it into that shape and most people hold the "trunk" stable with a garden stake next to it. Being in California I bet there are a lot of roses you can find locally and grow them. In my garden I have a small rose bush that I got from just sticking a broken rose limb into the ground, so some can be propagated cheaply that way.
On this PRI companion plant list you will see roses next to the tomatoes, chive, onion, parsley, asparagus, marigold, nasturtium, carrot, garlic, bee balm, so any of those would be considered good companions to roses.