Lydia is the level you first meet her at, i think you can fix this to make her level to your level with the wabbajack if she turns into a rabbit. Or with the pc, disable her , then enable her. She is really strong, but you have to give her better stuff. Shes not very sneaky, for that she sucks pretty hard, but give her a good sword and shield she is awesome. Aela is only killable after you progress so far in the companions questline, so if you want her to be invincible you cant finish the questline, you can marry her as soon as she becomes an option, she will still be invincible until you progress the questline. Put her in light armor and she will be your best sneak follower. My favorite follower is Mjoll, she is invincible, no matter what, give her a 2 handed and heavy armor she will wreck everyone, or give her a shield and she will use grimsever ( super OP ) , again put her heavy armor. Another great follower is the mercenary in riften, for a mage character he is awesome, two mages will dominate anyone, if your a tank, he is really helpful, for a sneaky character he is still alright because he wears clothes. Its definetely Mjoll all the way though, once you kill her creepy friend, i have married her on most of my playthroughs, other than the times i made sneaky classes and i married Aela. Just keep in mind that they are the level of when you first saw them and their max level is 50, i believe for all followers, save first, then use the wabbajack.
A little side note, fight Lydia, level up your stuff a bit, dont die, dont kill her, run away, she will respawn at either your house or dragonsreach, she will no longer be hostile, you can also use calm and teleport out.