Please no major spoilers on these.

 

When you fight with each of the Sith Lords are you fighting Solo like with Malak in Kotor 1? Or with a party?

Some you fight solo, some you fight with a party. That good enough? <_<

Which ones?

Those ones, over there, are the ones you fight as with a team. The ones here, here, and here, are fought alone.

 

Other then that... play the game and find out. Unless you want to know who you run into when.