Well it seems best to make a list of what I think is right and what I have read and we can use that as a starting point, not everyone may agree at the end but I think we can determine an optimal set of 6 toons to use strictly with KISS that will work well together and can run unattended for long periods.
Bard - What KISS was originally meant to run, 100%
Beastlord - Good steady DPS, lots of utility to back up tank or slower, 85%
Berserker - Decent DPS but more of a burn type, 70%
Cleric - Best healer and needed for rez, 100%
Druid - Like a bard, huge utility, ports, heals, dots, DS and evac, works well with KISS but may burn mana fast?, 95%
Enchanter - Best mez and slows but poor DPS, loses to a bard except in extreme circumstances but works well with KISS, 80%
Mage - Works well with other pet classes, best DS, back up tank with earth pet and CotH, works well with KISS, 100%
Monk - Decent DPS but too complicated for KISS, one dimensional as far as a bot group goes, 80%
Necromancer - Good DPS but a little complicated for KISS, back up rez, 85%
Paladin - Good single target agro and can heal itself but for groups SK wins, works well with KISS if you like the class, 80%
Ranger - Pretty good DPS as a ranged, very gear dependent, simple to play and works well with KISS but very one dimensional, 85%
Rogue - Good DPS but too complex for KISS, very gear dependent, 80%
Shaman - Slows, buffs, dots, heals, rez, works well with KISS, 100%
Shadow Knight - Best AE agro, little complicated to set up tanking for named mobs, good puller, snares, works pretty well with KISS, 95%
Warrior - Best for named mobs, pretty easy to set up, little utility, works well with KISS, 90%
Wizard - Ports, big burst DPS for named but too complicated for KISS, 85%
So for must haves it looks like bard and cleric for sure, then you have to choose between a pet tank or hybrid tank set up. Pet tank team would be bard, cleric, druid, mage, necro, shammy (or beastlord?)to give you the strongest pet, tons of dots, lots of back up heals for named, mez, buffs and ports. Melee tank would be bard, cleric, druid, mage, shammy (or beastlord here as a better fit than pet tank team), SK to give you everything and back up for every role.
This is purely from a balanced for KISS standpoint where you would want to get everything running smooth and just leave them to do their thing.