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.