Is it possible to mix the two types of switches in the same stack? Can I insert a 3750-E into an existing stack of 3750 switches? Will I lose any functionality?
yes you can but it reduces the stackwise capabilities, please see the following
Q. Can I run Cisco Catalyst 3750 switches with Cisco Catalyst 3750-E switches in the same stack?
A. Yes, the two series are compatible. When the Cisco Catalyst 3750-E Series detects a non-E-Series Cisco Catalyst 3750, it will negotiate down from StackWise Plus mode to StackWise mode. Thus, the entire stack will now run in the original StackWise mode.