I just reached layer 4, I think, and Selina joined my party. And, as it looks like, I have now free control over my party setup from now on.

But I wonder: Can I just take the characters I like to use from now on or am I screwed if I do so? I mean, are there some plot events, where I'm forced to use a certain character and end up screwed, because that character is underleveled?

Ah, that's very good to know. I mean, that inactive party members level up as well.
I noticed that several characters level up a lot slower, though.

Thanks a lot!

A lat depends on how much you fight with early characters though. If you breeze fast enough, Grubb will be the weakest character in the game for example and Maya will be rather medicore. Than if you fight a bit on next shell Corgan will become the most powerfull group member and a tank, etc. When I fought a bit early, Grubb was medicore fighter and powerfull spellcaster (compared to whole character rooster) and Maya was overally just over powered.

Keep in mind that next characters that join you, often (always?) join on fixed level. That is why it happens. And that is why Corgan can either be overpowered or a weakling.

Characters in active party develop much faster too. But no matter how you play, the game should always be playable for you. In worst case scenario you can always grind a little before particular boss.