It depends on how the aircraft is certified. The KA300/350, BE1900 and SA227 are certified for single pilot operations.The BAe3100 is certified for two pilot operation only. In single pilot aircraft requiring a type, the pilot may get a regular type rating permitting single pilot operation, or get a type rating with a SIC restriction requiring a SIC. To get a type without the restriction, all you have to do is prove you can operate the aircraft in all phases and situations without a SIC. Easier said than done. Most 1900 and Metro type pilots have the restriction as the aircraft in passenger 135/121 operations require an SIC.