In the Panel From Hell interview, toward the end iirc, Swen stated that there would be more companions, races, classes, and features introduced throughout Early Access.
5e is also really flexible in that you aren't limited to absolutely needing a certain class (or in this case, bringing along a certain companion) so even as someone who prefers to hoard companions throughout the game and would even prefer a bigger party option, I don't think locking companions after Act 1 will have us feeling boxed in with few options or risk losing xp/loot.
The game will allow you to hire mercenaries. In DOS2, the feature becomes available in Act 2. I don't see why it would be different here. The "it limited the party skills/options" isn't actually an issue. Sven even talked about allowing people to make full custom parties.
As for "locking" the party after Act 1, I personally will wait to see what happens. There is already quite a few possible scenarios here, starting with "couldn't save everyone from ceremorphosis" to "some joined the Cult of the Absolute while others didn't" (there are a few dialogues in the demo presentations Larian showed that suggest the Absolute leaders also have tadpoles).
Or Raphael kidnaps the party and send it to Hell (again)...
