Rip Dere Heads Off doesn't specify I have to use it to target one of my developments, so can I choose an opposing development (usually in a zone I won't be attacking that turn) turn it face up and sacrifice it at the end of the turn? Or can I not sacrifice opposing cards, so they'd end up with a unit? Or can I just not choose one of their developments in the first place? Anyone got any opinions on this, because if I can target their developments as well, Rip Dere 'Eads Off! becomes even more solid

Unless it says otherwise you can target anything with a card effect. So yes the player would be forced to sacrifice their own development which is more reliable than killing a unit or destroying a card since the rules specifically say that sacrificing can never be prevented.

Yes, I use it on opponents occasionally. It is allowed.