I don't know how old your girl was when she was pulled from her mom but it is true that if kittens 'leave the nest' too early they may have missed out on some of the socialization and training that normally occurs as kittens grow up, including litterbox training. Cat Attract or Kitten Attract usually works exceptionally well in these situations. Also, make sure that the litterbox is never too far away, in other words when she is exploring the house she has easy access to a litterbox so that she doesn't decide to look for a more convenient place to go.
As for the differences energy levels between your older cats and your kitten, this is not uncommon. Try giving her some dedicated play time - pick two or three times during the day/evening when you can play with her and try to stick to the schedule. Cats actually like routine and do well with it. If she knows that a play time is coming up she may be more merciful on the other cats. In the meantime the older cats will let her know when they've had enough. Unfortunately, sometimes a Savannah can just be overwhelming for older, less active cats and they need to be separated but I would give them a chance to work things out on their own first.