I wouldn't want companions spaced out between the acts, personally. I don't see what the benefit would be.
If some companions only become available late(r) in the game, odds are, by the time you get to them, you won't need them. You've already worked out your party composition, decided who you liked personally, maybe even got invested in their storylines – a newcomer doesn't stand a chance. They're more likely to get benched and ignored for the rest of the game, save for their personal quest.
Plus, having less options for constructing your party from the beginning, you may feel pressured to play a certain way or a certain class to plug holes. It's a problem BG3 already has, and spacing out companions will only make it worse.
The premise of the plot doesn't seem implausible to me either. You've all been abducted by the Mindflayers, you've all got tadpoles. Sticking together will raise your chances of survival, so you stay even if you don't like each other. You can't have outsiders in the party – you can't keep the tadpoles a secret forever, and without the threat of their own tadpole hanging over their head, any outsider is liable to try to murder you because by all accounts you're a danger. Makes enough sense to me.
