I just got my Ipod and i have a few question

1) Do i have to keep every track i want on my ipod on my pc at all times as i just had a problem. I added a couple of tracks to itunes and then put them on my ipod. i then added a few more albums but i did not want to add them just yet so i unchecked them aswell as the other albums i already added. now when i connected the ipod all the music i already added was now gone from it. I am assuming it was because the ipod tried to sync but as nothing was checked it did not find anything to sync. Is there a way i can get around this so i don't have to keep every cd i rip on my pc.

2) Is my sister going to now have problems with her ipod. She lives in america but bought an ipod while she was over here for christmas. now she used my pc to add tracks to her ipod (from cd) for the flight back but now i am reading that you can only use 1 ipod on 1 pc and 1 piece of software. Does that mean that when she goes back to america she will not be able to use her ipod on her own pc with her own copy of itunes?