You are correct. Persue one bird at the exclusion of the others. Also, the elective classes do matter. Having more points in one area or another will affect the ending you get if you follow certain birds, and answering certain questions one way or another may also affect the ending.

Two things I can advice you with and not spoil things too much but save you some trouble...

First, if you follow Anghel's route (not possible until at least the second playthrough) you will need to max out one of the three stats to complete it. Two of the stats will allow you to succeed... if you max out the one incorrect stat or fail to max out one of the correct ones you'll fail no matter what choices you make at the end. It's generally a good idea to save often in any path as sometimes making a wrong choice can get you killed in some of the paths.

Second... at some point during replaying the game you will be asked if you want to fulfill a promise or live a normal life this semester. Hold off on fulfilling the promise until you have all the ending achievements besides "I'll be Waiting" and "At the Dawn". That will save having to play through that path twice if you don't want to. Also... don't choose fulfill the promise until you can play for a few hours uninterrupted. It's important to get the full experience. You'll both hate me and thank me later for this. If that makes no sense now, it will when you're done.

Thank you! I will most definetly take your advice