I thought that you had to perform all your ATPL multi-crew time in aircraft with a certificated minimum of 2 crew, but I've heard of some pilots using time in single-pilot aircraft towards their EASA ATPL licence. Is this true? If so, could someone explain how is this possible?
According to the UK CAA you need:
500 hours in multi-pilot operations on aeroplanes;
Notice that it does not say anything about the aircraft being multi-pilot/certified for two crew members.
An air carrier can specify in their ops manuals that, for their operations, they require a particular airplane to be operated with two crew members even if it is only type certificated for one crew member. If their manual is approved, then it is now a "multi-pilot operation" regardless of the type-certification status of the airplane.