So for 1.5, for both KH1 & CoM, I started each game on the highest difficulty level during the first run. I would like to do the same for 2.5, but is this possible? For the final mix versions of KH2 and BBS do I have to unlock the hardest difficulty first, or will it be available from the beginning?

I believe you must unlock Critical Mode by completing Proud. 

Well the hardest difficulty in both of them is Critical and I can't remember if they are unlocked right away or not, if not then I'm guessing you either have to beat the game once or beat it on proud.

Yes, you have all the difficulty modes unlocked from the beggining. The only kh game that didn't have this feature was 3D, where you had to beat it one to unlock Critical mode. In every other game you have the the hardest mode when you first put the disc

I'm pretty sure that Kingdom Hearts 3D is the only game that makes you unlock critical. In the original Kingdom Hearts II Final Mix+, I think critical was available from the start. I don't know about Birth by Sleep, though.

I believe you must unlock Critical Mode by completing Proud. 

Um...the only KH game that actually had you unlock the hardest difficulty was KH3D. The other KH games had the hardest difficulty available from the get go.

Critical Mode is available from the very beginning. No need to worry. 

Thanks everyone! I wasn't fretting over it, but I just like my first playthrough to be my MAIN playthrough. Now I REALLY can't wait for 2.5!

Just like everyone said, Critical Mode is unlocked from the beggining

 

Good luck on your Critical Mode run