If I select a pre-made leader who has attached elements, the "split" button is available. Clicking that obviously splits the leader's elements into their respective parts, and then they can be recombined.
However, if I take a new leader and new elements, the option to combine is not available? Why not? What controls this? Whether the leader is active or not seems to have no bearing. The tooltip just says in red "you have this many units selected", then only one leader allowed, and only whatever else, all of which should be fine, and less than 9 total elements.
Another thing, when I build infantry, it comes with artillery?