Hi, I tried it a bit the other day.
THe fire breathing guys are quite devastating. At least vs the AI. I'm not sure, but I would probably pay the double and not feel ripped of. Compare them with salamanders. They have approximately the same dmg, but also range. Also more ammo and can hide behind troops and still attack. A bit vulnerable to missiles, but that is possible to alleviate.
Poisoned missiles are currently a bit bugged, and poisons regardless of dmg, which makes the darts quite good. But with 1 ammo it is no big deal.
I liked the traps
Commanders that summons troops for free are very powerful. If an ash wyvern is worth about 50 gold, each summoner has a break even point of 6 turns. I didn't bother to recruit other commanders, unless I was hard pressed to get a fire mage or wanted to try a summon.
Kobold summons are very cheap. 30 for 5 gems is even cheaper than longdead and other banishable chaff.
The medium inf are quite elite (and cost as such). Their skill levels are comparable with Wardens and other elite units. Might be intentional, but I would suggest renaming them so it is obvious that they are the elites of the nation.
The light inf is also well drilled by human standards. But you might have intended for the kobolds to have 11 as base skill level, being a warlike people.
The miners seems quite cheap and easy to recruit. For a few bucks and one month of recruiting you have a force able to take down most walls. I don't remember what similar units do cost in the vanilla game.
Didn't play long enough to test the summons.