matt is pretty dead on...
You can slap on whatever head you want really
If you\'re gaming with it though, it\'s really just a matter of, does the head represent a piece of wargear and it it legal gear for the unit? Even if the head represents gear the unit can not use, you haven\'t paid the points for it and as long as you don\'t try to use the wargear it represents, it\'s all good.
I.e. , you may have a winged Hive Tyrant model, but in a particular army list you don\'t pay for the wings ability, so you just use it normally. Again it\'s only an issue if you try to use the winged ability when you haven\'t paid the points.
At the risk of sounding redundant, if it\'s just a modeling preference and not a gaming concern, then it\'s a non-issue. Some purists may whine that a certain biological feature is not available to certain form, but telling a fanboi to shut-up is almost a hobby in and of itself.
:beer: