Just an update for everyone who gave advice!
Took a trip up to Dobbies today, and what luck! They had a load of shrimp species in stock, unfortunately no ghosts, but I took home 4 amano to try in my tank (had to stop myself buying up the all oto and cory cats, so cute!) along with a few new plants to provide heavier plant cover.
So far so good, Mordi was extremely curious at first, flaring and chasing them, but not nipping. He seems to have gotten used to them now, no flaring or indeed showing any interest in them at all. They look so nice in the tank I'm thinking about getting some more! Or maybe another species that'll breed, like ghost or cherry. And I've noticed they've nibbled some of the algae growing on my driftwood already, good sign!