I've been riding my roadie converted to fixed and I've had no problems with the rear wheel slipping. And I'm even using a steel QR! So there!

I really don't see a problem using a bike with semi-hor or even vertical drop outs as long as the tension is good. You are just limited in terms of cog/chainring combos that's all. But like Sohi said, nothing that a half link or White Industries ENO hub can't solve.

That frame looks like it would make an awesome fixie convert.