As has been said before, the beauty of Frostgrave is its so open, so I'd be inclined to make up my own stats to suit the figures.
There might be an issue with the limited stats available ie Move, Fight, Shoot, Armour and Health so getting enough variation to make a kobold different to a goblin might be tricky.
Having said that some house rules will help and also letting creatures have 'built in' spells could give you that variety.
I've been considering specific demon types myself to make them a bit more interesting, for example a Succubus would be as a normal minor demon for stats but would have an automatic Beauty spell in place with a cast roll of say 14 (based on the spell cast + the demon's will of 4)
Thinking about Kobolds and the fact they are reknown for the skills at ambush you might let them roll for fast act at the start of each turn needing an 11+, if they pass they get to go first?
Just thinking out loud, not tried any of this.