I’m not quite sure what you mean here. HISM can be used with actors which are dynamic (see: all particle emitters). If you’re placing hundreds of rocks and each rock has an LOD, you could use HISM, but it would take a bit of extra work upfront.
I’d probably create a blueprint which has a list of all the different rock types (array of meshes), and then create an array of transforms for each rock type. So, rock type 1 has a corresponding array #1, rock type 2 has array #2, etc.
In your construction script, you’d run through each mesh template and place an instance at each array transform for the corresponding mesh.
It would be a bit clunky to use in the editor, but it’d work. If you need the rocks to move (such as in an avalanche), you can go through each instance and update the transform position per tick.
Alternatively, the better solution is to just let the foliage tool in the editor place your rocks. It would handle the placement and back end management of each rock and give you good tools to edit and customize each instance. This is good for rocks which don’t move though, so rock avalanches would be out.