It's absolutely wrong to think that more characters in parties absolutely means more ennemies...
BG1/2 (and nearly every games...) has many encounters against 6 opponents, but also versus only 1 or 2... The number of opponent is not what define the difficulty
And it's absolutely not necessary to rebalance everything because you add 1 more companion... Every combat doesn't have to be a "main event" after which you have to rest, use all your heal/spellslots or potions or even worse : rest to deal with the next one...
At least, one more companion would totally rebalance what actually don't work in the game and won't when it will be release. (resting after each battles, very slow combats, lack of variety in parties, extreme difficulty of some combats in this "normal" game mode, help companions that fall on the battlefield everytime...)
