Use riding rules, but in combat you can only fit as many as there's confortable space. You're considered ill suited as a mount.
So it's a DC 5 check to stay on when you take damage, you can make a DC 15 check to take cover (at the expense of any other action) and mounting/ dismounting is a move action unless you make the DC 20 check to make it a free action. All of these checks get -5 because Regideus is ill suited as a mount (until he takes a level of Titanic Creature anyway.)
For mounted casting, use the mounted combat rules (you can cast before or after the mount moves without penalty, to cast while moving make a concentration check DC 10+ spell level (does that take the ill suited as a mount penalty?))?
Because the guys in front are being used as meat shields. You know, actual reasons for people to make tight formations.
In front of what? An area effect that encompasses the whole of the mob's area would hit the guys in front, and the guys behind, and the guys anywhere else. That's kinda the point of area effects (and the reason that tight formations don't protect against things like artillery, poison gas, flash grenades, and similar analogues to magic AoEs.)
In any case, what about effects that do things to the ground, rather than the members of the mob itself? For instance, the Earthbreaker formula (predictably) breaks the earth, which immobilizes for a turn and makes the ground difficult terrain. Would that affect a mob normally?
"If a mob fails a save against a spell/effect that would normally kill/incapacitate a certain number of creatures, such as disintregate, slay living or wail of the bashee, it loses the total HP of the dead members from it's HP total."
So what does an area daze effect (which would incapacitate everyone in the area) do to a mob? Destroy it in a single hit?