I was trying to get over to an area where the yeti guys were and they kept block my path and I wasn't able to get over to their quest because their yetis kept blocking my path.
I'd suggest that neutral units don't block movement. It would help a lot. If you are worried about cheating then just make it where you can only cross if you have at least 2 movement points and can't end your turn on top of a neutral unit.
As for allied, well, I think you should be able to stack on them all day long and even fight defensive battles with them but I suspect that's a lot more coding.