The critical issue for using custom R/D/T for formulaic spells with just ArM5 p.114 is, that it
... is largely left to storyguide interpretation.
So T: Pair may look universally accepted as a custom Target, while T: Road may rather look like requiring either Faerie Magic or Mercurian Road Magic.
But a more eccentric SG may allow T: Road anyway by p.114, and not approve of T: Pair at all as a matter of taste.