Mustalʾimīn is a masculine sound plural in its oblique form; in context it describes an assembly equipped with armour. The point is not just the presence of soldiers but their readiness for conflict.
Kabsh is a metaphor for a leader: no literal ram is intended. The separate expression ka-annahu ʿablāʾ is an explicit simile. One image implies commanding prowess; the other makes the leader appear elevated and conspicuous.