Oscar is a dwarf and gets insecure in open spaces - you might find some feel insecure in a large cage, but it's not a problem if it's filled with toys.
A rabbit cage is great for a hammy as long as you take care to mesh it completely to prevent escapees. They can climb but are quite clumsy so, when adding levels, it best to design the layout so that there aren't any long drops so add things like cargo nets etc to break the fall (just in case hammy decides to jump from the top floor).