Thx.

Well the long way round is to have a block containing "missile infantry" with low priority and another one containing only "ranged_missile_infantry" with higher one, so that skirmishers will end up in the first block. It's not a perfect way, but it's acceptable. While now you can use "skirmish infantry" and it will work perfectly. I know vanilla, Sin and Darth already use it, but they always use it along with "missile infantry" for their interception-line-block and they use "ranged_missile_infantry" alone for the archer-block, so that skirmishers always end up in the first-line. I made a custom formation not a week ago and I had assigned only "skirmish infantry" in the first block with a minimum of two such units. I gave the formation a high priority (it was a defensive one, so it would be selected for deployment) and made sure all the other requirements were met, but guess what: the formation was never chosen. When I changed a unit's class from missile to skirmish, the formation was selected.

Note that perhaps the class isn't fully supported (I can't understand why it wasn't included in vanilla otherwise) and it needs further testing.