As far as servers go it is possible, MMOs are designed around fighting together so latency needs to be as low as possible anyway.

The problem comes (talking specifically of FFXIII) with dealing 10 million damage in 20 seconds, as long as your dds could build a chain (will be no problem with a decent tank) fights would be so easy, the reasons it works in FFXIII is because it can be tough to build a chain on stronger bosses because if your CON or ravagers are taking too much damage you need to swap to possible SENT and MED to heal, and while you are healing the chain bonus is dropping. In an MMO this has no effect, healer tank and all dds, you would stagger most mobs very fast without the need to change for heals or w/e so just stack the classes that chain fastest (possibly one that attacks fast to keep the chaing guage high) and all mobs will drop like flies unless they have over 1 billion HP.

You may say "well limit party sized to 3 to avoid this" but then why not just play any offline RPG.