Ask HN: Is it okay to do part-time consulting while working a full-time job?

Background: I'm deciding between a job at Google vs. a startup. I'd really like to take the job at the startup, as I found the people and the work much more interesting, but at Google I'd be making about $100,000 more from bonuses and stock every year, and my family kind of needs the money right now. (It's possible that I'd make more money from the startup in the long run, but it's obviously uncertain. To be clear, both companies are offering a $100,000+ salary, but my family is going through some pretty severe financial troubles that eat up about $5,000 every month.)

So I'm wondering if I could take the offer at the smaller company, and simply do a small amount of consulting (5-8 hours a week) on the side to help make up the difference. Is this normal/okay? What are the rules around this?

The consulting wouldn't affect my main job, and they would be in distinct areas of work. Is this essentially "unofficially okay" (do people do this all time and no one cares as long as you don't talk a lot about it publicy)? Is it not recommended at all? Or is it even pretty officially okay (so that I could, for instance, state on my website that I'm looking for part-time consulting, without worrying about consequences)?

Update to add: in my offer letter from the startup, the non-compete clause essentially says (paraphrasing)... Duty not to compete: My work at the company requires my undivided attention. I will not, without the company's written consent, engage in any other business that directly competes with the company, uses any of the company's materials, or otherwise conflicts with the company's business interests.