Following the vague, confusing crystarium guides is ruining this game for me. Still, I keep seeing posts on many different websites basically saying that it doesn't matter, and everything in this game is easy anyway. So is there any penalty to developing characters haphazardly? Will it make it much more difficult to beat extra content?
It won't be too bad. I leveled up haphazardly my first playthrough and platted the game with ease. Strategy over stats, imo. That said it's not THAT difficult to figure out the crystarium, and you don't need to follow a guide to get the max in the particular stat you want. All you need to know is how the stat bonuses work. Every time you gain a level on a large node you get a small bonus depending on what role you leveled up in:
COM-Strength +2 RAV-Magic +2 SEN-HP +6 MED-HP +6 SYN-odd levels (going from 50->51) Magic +2, even levels (going from 49->50) HP +6 SAB-odd levels (going from 50->51) Magic +2, even levels (going from 49->50) Strength +2
Otherwise both Serah and Noel get the exact same base stat boosts from every single node on the crystarium. For example, on a large node Serah gets HP+6, Strength +3, and Magic +2. That never changes no matter what role you choose to level as. The only thing that does change is the bonus you get from the role you chose to level up as that appears after the base stat increases show.