Can a non psv driver driver a small psv vehicle with less than 8 seats?

Can a non psv driver driver a small psv vehicle with less than 8 seats?
In the UK? Yes. Providing the holder of a normal car licence held that licence before 1997 they may drve a minibus up to 17 seats - but not for hire or reward which would tend to discount PSV.

Drivers without minibus entitlement on their licence may still be entitled to drive vehicles up to 8 seats, but not for hire or reward which would discount PSV.

More information at: http://www.direct.gov.uk/en/Motoring/DriverLicensing/WhatCanYouDriveAndYourObligations/DG_4022547

As the answer to this question may have serious legal ramifications if it is either incorrect or misinterpreted you should seek clarification directly from the licencing body for your country. In the UK this is the DVLA - telephone:
0300 790 6801