Rhindon - i'm not sure about other game types, the tutorials advise in platformer's for your player at least to have an invisible player box with the platform behaviour, and working as your collision mask, and then sticking your animation on top of that box with either pin or set position (i'm not sure which is better, if either is even).
the reason i mention this is it follows the same principle, you can change your animations but your actual player (invisible under box) remains unchanged and therefore always interacts with the environment in a consistant manner.
you could use image points to attach other invisible hitsensors and use that instead of the x and the y of the sprite's to weed out even further changes.
(again i'm not sure which is the "better" approach pinning or setting a position), you could just make sure that it's there - by pin or set position - and then on collision or on overlapping the to-be invisible hit sensor take damage. if you change the sprites size later on you'll just have to change the image points and the hit sensor sizes instead of the code.
it's not that big of a deal either way, just something to think about <img src="smileys/smiley1.gif" border="0" align="middle" />