I've been a huge fan of the online pvp aspect.
1) the classes that stand out the most are the ones that are capable of fulfilling a role better than others. Thus we have:
Swordmaster/ Elite Ninja (dodge tank)/ Falcon knight/ Dancer (support)/ Berserker/ Wyvern Lord (ORKO (one round KO))/ General (anti-ORKO)
2) Diversity in a team is good if your team is cohesive because of it. Having multiple of the same classes even can be a good choice.
3) Right off the bat, you can expect to see a lot of 5 man ORKO units. A balanced team could run 1 swordmaster/ Falcon Knight/ General (fill)/ 2 ORKO units. And if ORKO units are rampant feel free to use less diversity in your team and commit to an anti-ORKO team using multiple Generals.
I could go into more depth but I'll leave this here for now.