Kangaroo can find a key in any range, regardless of its size. Even if you specify the wrong range, kangaroo will still find the key, but it will take much longer.

JLP's kangaroo implementation does have a problem with small ranges because by default there are too many kangaroos in a small range. You can adjust CPU/GPU group size to fix it, search with cpu only, or search with less threads/grid size.

Regarding searching for multiple keys, the kangaroo algorithm only works with one key at a time, so such a search is only possible sequentially. If it were possible to search for several keys simultaneously without loss of speed, then the puzzle's addresses with revealed public keys would have been solved long ago.