"Cisco Catalyst 3750-E and Catalyst 3750-X StackWise Plus and Cisco Catalyst 3750 StackWise switches can be combined in the same stack. When this happens, the Cisco Catalyst 3750-E, or Catalyst 3750-Xswitches negotiate from StackWise Plus mode down to StackWise mode. That is, they no longer perform destination stripping. However, the Cisco Catalyst 3750-E and the Catalyst 3750-X will retain its ability to perform local switching."
Right, die stacking-bandwith will be reduced from 64Gbps to 32Gbps.
But more interesting is the Patch-, Softwaremanagement and the stability of the stack.
The bootvar is different, So you can not use the "archive download-sw" command to update your stack because you have to use different tar-files. You have to disable the auto-copy-function. What is with bugs? Fixed in the X-series-IOS but not in the "old/normal"-IOS... and now?
So I don' think, that it is a good idea to use a mixed stack.
If your plain 3750 is using, for example, IP Services feature set then you can get an IP Services feature set for the 3750E/3750X. I wouldn't recommend using the "universal" when you are mixing them together.
Another thing, 3750/3750G with 16 mb of flash is only limited to 12.2(55)SE and later. You can't use 12.2(58)SE or 15.0(1)SE.