You can stack any 37xx with any other, although you cannot use the 3750G-12S models with other 3750s if they are using the SDM templates unique to that model (they can be configured to use the "common" SDM templates).
When mix StackWise (3750 and 3750G) and StackWisePlus (3750E and 3750-X), the stack operates in StackWise mode, losing the advantages of StackWisePlus. If possible, I recommend you don't mix the two technologies.
The 3750-X also supports StackPower, which is unique to that series of the 3750.
Leo mentions you should investigate IOS version and feature set. The stack software must be compatible (easiest to accomplish if all units run the same IOS version) and although you can mix feature sets, your stack may behave differently depending on which stack member is master.
Earlier 3750s do not have enough flash to run the later IOS versions. I.e. such a unit might not be able to stack with later 3750s running later IOS versions.
With the 3750 series, you do not need to buy any additional stacking module, stack ports are built in.
3750s came with one short (50CM?) stacking cable. Longer stack cables had to be ordered separately. (NB: the short cable is generally long enough to go vertical a unit or two, but larger stacks, to go top to bottom would need the longer cable. However, "braided" stack connections can avoid the need for a top to bottom long cable.)
