Just don't pick axe main, and it's all right
FYI cannonM + artillery (Deep Insight + Devastation shell) gives a massive 5k+ defense debuff. For non-cash shoppers, this makes PvE a lot easier.
Like Miyu said, each class is unique. I think you'll have enough time during the game to decide which formation suits your play-style best, as long as you ask a lot of questions.
Just make sure you put the correct balance of attack and support. Early on, a shaman and monk is enough. You'll have plenty of time to learn about prophet, princess, exo and other unique/versatile C-mercs and decide which ones are best for you.
You'll find every type of main chars in division 1 PvP.
Cannon and guitar remains as ABFL favorites.
They mainly run a full ranged build usually with a princess. I saw Geemarc put a hwarang at the front e_e
tbh cannons are becoming fotm in Titan atm :v
Best Titan melee classes are still Sword and Power Saw.