Fundamentally, I don't see an issue with you doing outside consulting work, as long as it isn't in direct competition with your main employer. I would definitely never mix assets - e.g. don't use your main work computer for moonlighting work. Moonlighting has been around forever, in all trades, and won't go away.

Realistically, this is a very difficult task to pull off in our discipline. This was true for me anyway - I've tried it and it was short lived. It could be that I'm an "all or nothing" kind of guy and it is just difficult for me. Maybe other people don't suffer from this quirk. For me, I find it incredibly difficult to shift between simultaneous projects. It takes a certain amount of mental time just to get into productive mode. By the end of the day, I'm pretty exhausted from my main duties. What happens, then, is I ended up doing okay at one task, and terrible at the other, but didn't produce the kind of quality output I would want for either. Ultimately, I wasn't pleased with myself.

The startup is going to expect you to be 150% focused on their objectives. I don't think it will be an 8 to 5 gig. That's not the way these things work. It's a heck of a commitment, and if it were my startup, I would want you "fully engaged."

Just my 2cents.