if you go around with 10+toons in an area, you'll run out of mobs to kill very quickly and spend most your time waiting for the new spawns, or travelling to a new zone to clear i reckon.
Also, you'll need some 'main' toon to be able to somewhat finance the rest of the group. killing mobs with 10+ (let alone 25) will not net you with alot of gold per toon, even if only focusing on the 5 in the tag-group. eventually you'll not be able to buy all your skills as you level.

if i were to do this, i think i would either
- level 5 separate groups of 5, each with 4 shamies + 1 other class (healer or tank for example) and grind those groups of 5 toons in instances.
or
- level a first group of 5 to 70 as fast as possible, then use those new 70s to each boost a group with 4 new toons.

my 2 coppers