Pathfinding is hierarchical, tile is the smallest element, and several tiles can form an area called cluster. Groups do long range pathfinding usin cluster data, while units do short range pathfindings using tile data. Moreover, an imaginary unit is used by the group to define the group middle tile path among clusters, used by the formation system.
The formation system is written from scratch, and uses a matrix like database and calculations for defining target tiles for each unit. It is quite a complex stuff, I would need pages to describe it… Maybe once in a blog post.
Unit collision is also a custom system, using priorities, and short ray traces for push/bounce.