I have used DG's in 2mm and 4mm.
In 2mm, a loop on one end only worked best as two loops coming together at exactly the same height just resulted in the wagon being pushed away.
In 4mm I had no issues using loops at both ends, the extra mass of the wagons helping. I would imagine that your 7mm stock would be even better.
I did blacken the whole brass fret before assembling the couplings, just touching up any odd shiny bits with a black marker pen after assembly.
Rob