Sure. The kobold is weak enough that an extra trick or two won't
overpower anything. Who knows? It might actually bring the creature up
to to the level of a gnome.
However, an area effect weapon is still hard to control for the DM,
especially at low levels. A better choice might be to model the
ability around the _Fire Breath_ spell in CArc, which is limited to one
target per breath. Plus, they need some sort of fire resistance. With
the low hp of a kobold, its a wonder they don't kill each other off
everytime they sneeze! Finally, letting them breath fire more often
will not really hurt anything, as they will still likely only breath
fire once per battle.
The write up: Once per 2d4 rounds, a flamekin kobold can breath a gout
of flame as a standard action. You make a ranged touch attack with the
flame (up to a range of 15 ft.), dealing 1d8 damage per two character
levels (maximum 10d8) on a successful hit. The flame ignites
combustibles and damages objects in the area, and it can melt metal
with low melting points (such as lead, gold, copper, silver, and
bronze).
Flamekin kobolds have Fire Resistance equal to 5 plus their character
level (for example, a 12th level flamekin kobold has fire resistance
17).
>> Stay informed about: would this be somewhat unbalanced in a game