Bindonblood,
I'm a huge admirer of your work! I would humbly say you don't necessarily need to wait for your other unit of Highlanders, as the officers of the 72nd are known to have worn PUTTEES with their trews (made from matching tartan pattern material, which makes for a pretty cool & unusual looking detail), so you can easily use the already available British line infantry officers as HIghland officers of the 72nd, while the pipers of the 72nd actually wore KILTS, so once the kilted command pack is available, there's your piper for the 72nd as well. It's true, drummers for the 72nd probably wore trews, and mine does (I used the Perry Sudan British infantry in Indian service dress for the entire unit). Of course do whatever you wish, just wanted you to have a bit more info!