Trust me. You don't want to go through the expense of starting a hostel and then get dodgy about shuttle insurance.

Do what the Hiker Hostel is doing - shuttle guests only and be covered by your hostel coverage. If you're shuttling non-guests and get in an accident without the right insurance you're screwed. And insurance companies have seen it all - if you try to say everyone is your guest because they're in your vehicle or some other obvious attempt to avoid having proper insurance they'll figure it out in about 30 seconds.

I'm pretty risk averse when it comes to protecting my assets but I don't think it's paranoid to be properly insured. Talk to pros - don't take the advice of laypeople dispensing free advice about how to run a business.