Those are optional bosses, right? I haven't played Final Fantasy X in a looooonnnnnngggg time. I beat it about a year and a half ago so I don't remember a lot other then the story.
No, they are required bosses.

Nemesis and Penance are the first two bosses you fight in FFX. Tidus does around 50 damage at the beginning, and Nemesis/Penance have over 4 million HP. Don't worry, the game mechanics work so that you can attack them once every five turns, so exploit that game flaw and fight your way to victory! YOU CAN DO IT!

(SPOILER)No, really, they don't SEEM like optional bosses? You can't tell that doing 99999 is NOT a part of the normal game? What normal boss (even final ones) have over 1 million HP, even in Star Ocean III? I REALLY don't think that going through the normal progression of the game involves ANY HP amounts over 4000, and as much as you haven't played the game in over a year, I think you'd still have the ability to discern which boss is normal, and which is completely optional.