That guy sure wasn't using a Burley Nomad.

I just got one, and have carried far more than 50 lbs yesterday and today, there's barely more braking force/distance required than there is with the bike alone, or at least it seems that way. The trailer is not even detectable in turns, it's like it's not there, although in the streights there's a sort of "surging" you'll notice, not much of a problem but it's there.

I'm running Conti Gatorskins for tires, they seem to be a long-lasting compound so with my trailer full 0' goodness 100 lbs or so and those on my rims, if anyone slides it should be me. And not a hint of a slide, anywhere.

Now, as to the single speed, I think it's possible, just gear it nice and low and be prepared to spin when you don't need that low gear, the gearing will have to be determined by the hills etc not the flat parts of the road.

Get a Burley.