There is no stats caps for any stats. You can go as high as you want or as the gears available allows. The question then is what do you sacrifice in other stats to gain high of a specific stats but it only really applies for secondary stats like parry, determination, crit, skill/spellspeed. For main stats like Int, you want to stack as much as possible. Your friend is wrong in that when comparing BLM to SMN, the differences depends on the interval. If it's burst damage, BLM wins. If it's sustained damage, SMN wins. This is assuming similar level of expertise in the job. Int does not translate 1:1 to Fester damage. The higher you get it, the more damage you do. For your last question, yes, it increases your DOT damage as damage is calculated based on your main stats and potency. With the same potency, the higher the main stats, the higher the damage you get.