Will mounting a hdd upside down damage it, in anyway. I will just be easier if I mount it this way, for cables and stuff
Nah, no problems there mate. Unless mabye it's really old, but I can't see it damageing anything made in the last 5 years.
Anything with fluid bearings should be okay, so just about anything made in the last 4 years or so. Just don't move it upside down when it's spinning
you can mount it anyway so long as its flat/level, if the width of a platter is at an odd angle it wont do it much good.
the platters can be at any angle they like, so long as they are steady, and you dont try to move the drive when its spinning.