Does this advice apply to fragging a green tree leather (soft and squishy sps)?
There are no soft and squishy SPS corals SPS stands for small polyp stony corals. Corals without a hard stony skeleton are generally referred to as soft corals in this hobby.

Leather and or tree corals look best when mounted perpendicular like a terrestrial tree. One of the best ways to do this is after cutting the frag push a toothpick through the stalk and then either glue the ends of the toothpick to a rock or wedge one or both ends of the toothpick under a rock, either way so that part of the coral contacts the rock.