You said that "Whoever's available to one, MIGHT not be for another" so there are love interests that will be available for more than one protagonist and that you have to decide which protagonist will get that love interests?
Or each protagonist will get a set number of love interests and there will be no overlap between characters?
It's monogamous, so only one LI per MC. But the way the acts in the game are structured, it's fairly easy to play the other routes (you don't necessarily have to start from the beginning, just before the choice.)
But as of Episode 1, you don't need to worry about locking in. Just get to know the LIs for now.