A startup is a business, not a charity. Don't kid yourself that freebies are pro bono in any sense whatsoever. To put it another way, giving these people free legal advice makes as much sense as them just giving you stock based on friendly feeling.

Charge your regular rate or don't do it. You don't need clients who don't value your time enough to write you a check. And you damn sure don't want them to tell all their friends or that big future project that also doesn't pay. Work with professionals.

Good luck.