"and not only are employee non-compete contracts generally not binding in California, but also it's not the nature of an enforceable non-compete that you can't do software development for any company (or even most companies)."
Locality aside, non-compete isn't what would get you here.
For most companies, they likely have specific policy in place - that you agree to around taking outside work while still employed by them. It might be they allow it, in which case it's fine. More typical is that you need to get some level of approval for work that doesn't conflict. In some cases they ban it outright, though I suspect the latter isn't particularly enforceable.
Certainly I've been in situations where I've picked up non-conflicting side contracts, and my employer has turned the other cheek because the approval process was tedious. However, that will vary significantly by company, and - legality aside - IMO they would've been within their rights to call me to task for it, because I knowingly agreed to their terms on the matter.
"Employers of salaried employees generally don't need cause to fire employees"
That's only so much handwaving. Yes, they generally can fire you for any reason or none at all. On the other hand, they typically won't. Unless you give them cause by taking an action contrary to the actual terms of employment you agree to. Then they certainly will do so.