Larian's silence on additional companions is indeed worrisome. Everyone keeps saying there will surely be more, but I'm not sure. And since the vampire spawn and Sharite should be automatic no's for a good-aligned party, party composition is absolutely set for a good-aligned party with no possibility for any choice.
More companions will be added throughout Early Access, so what everyone is saying is correct.
Having more companions to add to a party of four is still only a party of four instead of six.
Fair enough, but a party of six is less tactical than 4. In most games I play with a party of 6, I tend to find the sweet spot with my characters and I just go through the whole campaign only with one group. Having only 4 characters forces me into thinking ahead of combat encounters to decide whether I go more with a melee focused group, a ranged one or a magic centric build. It enforces decision making rather than getting the player lazy about choices. And I prefer that personally.