No.

Don't remember if that the case in all versions but, if my memory is correct, in BG1 if you add a new companion to your party for the FIRST time their level will depend on your own level. After you kick them out, however, their XP will not change as you gain levels.

The first time you meet them (recruit them) they will level up according to your current level. If you take them and then dismiss them, their levels will remain constant.

So, when you first meet a companion at level 6, they will (usually) be level 6. But if I recon correctly, there are plenty of XPs lying around the Forgotten Realms so leveling a lower level companion may not be too difficult.

Also, being summoned in ToB the non-party hirelings gains extra minimum level, AND you can gain infinite exp for them setting the AI agressive, and putting them on the city wall with a ranged weapon.

And just to be clear, the character who makes the finalblow to an enemy get the exp after that enemy. Any hireling in the party also gets the same amount of quest-exp when you gain those, and it's notasplit, everybody gets full amount.

Of course not.

I would like to add that there is some minor up-scaling when it comes to monsters. You might notice orcs were goblins used to be, or yuan-ti in place of trolls. XP is divided among party members, regardless if it came from a quest or an enemy.