Do companions have a fixed levels when I first meet them in game or do they receive experience based on main character's experience when they are added to the roster? Should I try to acquire companions as soon as possible and then finishing all the side quests/clearing areas to give them more experience? For example if I rush search for Bastila and then clear sidequests will she have more experience when I will finish Taris?
Once you reach level 7 any companion you get starts at level 6, but gets enough experience to level to one level below you. Before level 6, the companion is one level below you with one level up not chosen. So it does not really matter. At low level you will get slightly more choice in how you customise your characters, but there is no reason to delay leveling beylnd level 8.
There is one experience aspect not related to companions that is relevant for the decision whether to delay leveling (which will have impact on your companion level before this point):
After you leave Taris, you will start gaining levels as a Jedi class. Any levels not yet assigned will be assignable as a Jedi at this point. Some powerplaying builds will recommend leaving Taris with a couple of levelups banked so you have more powerful jedi skills at this point.
I have made two runs myself while I was offline and I think its results will be useful for anyone who will need more information. I have wrote down levels of my characters after talking to Bastila and returning to a hideout.
Run 1 - more or less straight for getting Bastila: All characters were at level 6.
- Main character had 19595 experience
- Karth had 17957 experience
- Mission, Zaalbar and Bastila had 15676 experience
Run 2 - cleared everything I could before winning a swoop race. Main character was at level 8, everybody else were at level 7
- Main character had 28165 experience
- Karth had 24165 experience
- Mission, Zaalbar and Bastila had 22532 experience
In both cases I have been levelling up Bastila to above written level from level 3, I had a prompt to level her up as soon as I had her in party outside of hideout. All levelups were manual, so everything was spent as I wanted. T3-M4 also gets the same level and experience as Mission, Zaalbar and Bastila after I buy it and can be levelled up manually from level 3. So most companions seem to always have a 80% of main character's experience.