Are Allen key or Security Key skewers appropriate for bikes with rear facing horizontal dropouts
It depends on how aggressive you ride. If you are a casual rider and rarely do many jumps or drops then they should work just fine. I am a pretty aggressive rider and have snapped more than my fair share of skewers trying to keep the wheel in place. I have also snapped a number of hollow axles because they are just not strong enough for the riding I do. So I switched to solid axles which are stronger (ultimately cheaper too). I have never had a problem with theft since I lock my both wheels with the frame.

If solid axles are not what you are looking for, I like the pitlock (that doesnt have the hex outer body), they can both be removed by a determined thief with a pipe-wrench, but the inactive side is smaller and harder to grip. Either one will hold the wheel in place about the same though.