The Cisco Catalyst 3750 series switches cannot stack with the Cisco Catalyst 3550 Series Switches.
The Cisco Catalyst 3750 series switches' new, innovative Cisco StackWise technology with a 32-Gbps high-speed stack interconnect is not compatible with the Cisco Catalyst 3550 Series Switches' GigaStack stacking.
The idea of stacking switches is for controlling or seeing a group of switches as one big switch. Similarly, the Catalyst 3560, 2970, and 2960 switches do not support switch stacking. However, you can still interconnect them in the same way you interconnect any type of switches so that they are seen as separate switches.
For concepts and procedures about managing Catalyst 3750 switch stacks, refer to Managing Switch Stacks.
For additional information, refer to Cisco Catalyst 3750 Q&A.