When we had 6 tanks in 24mans, there were tank shortages. When we have 3 tanks in 24mans, now there are DPS shortages. What if you just make the 8th slot in each party a "flex" slot that any job can fill? Tanks and healers have serviceable enough DPS options and 24mans aren't designed with strict DPS checks so it wouldn't significantly impede the ability of the alliance to complete the content.