Is it acceptable to ask about corporate culture in a less than 10 person company?

Are you interviewing with multiple people at the company? I would think you could pick up on the culture since you could easily interact with the entire company in a single interview (assuming it's a single office). If not, then yes it's acceptable, but maybe word it very casually.

Of course it's acceptable. You have to know about the culture and whether it's a good fit before deciding to join a firm (unless it's your only option). Even tiny firms have a culture. Definitely agree with above poster that it will be easy to pick up on during your time there interviewing, although sometimes people are overly nice and welcoming when talking to candidates and things can change drastically once you're actually working. Use your judgment on that.