So I honestly really like the idea of a kobold engineer, OC notwithstanding. But I just feel like it deserves better. My suggestion is retain the 0.8x size, 2x mining speed (or 3x or 4x like you said lol), and the 0.5x health, but give him a small 1.25x or 1.33x speed boost (kobolds are nimble too!), -20 inventory like the explorer (they're also small), permanent night vision effect (I mean, duh), and 0.75x crafting speed (they're usually not very bright and a little clumsy). It feels balanced to me and in-line with the creature while still feeling unique from the nimble explorer.
Also, in order to make the Solider and the Tank feel more unique from each other, I would suggest slowing down the Tank's regen to 2x or maybe even 1.5x (but still keeping it instant), and then increasing the Soldier's damage but decreasing his health to make him lean a bit towards glass cannon--the opposite direction of the Tank's lean, y'know? Though if you do that he should probably be renamed to make that lean more clear or people will see the health debuff and be like "huh?"