No you can Only go a New Line.
For Example you go to BB6 first then you reset the BO and go to SS Line you can only drive SS no more BB6 with your BO.
For your second question: You can switch the lines and only go to Ships your lvl is high enough. For Example:
Your BO (UK) is lvl 96 you go SS line and have SS 5, now you go BB line you can go to BB4 (L1 or Vanny) not to BB5. Beause you need lvl 103 for BB5.
