I'm curious about the the "double their hit dice and treat then as D10s"  rule for "monsters" - I'm picturing a tiny Kobold (e.g. D&D 5e, 2d6-2).Does it feel right to give her 4d10 hit dice, for an average of 22hp? 

What's the thinking here? 

"When you attack, roll under your stat on a d20 and deal the result of your roll as damage."

The average roll on a d20 is 11. That Kobold goes down in two hits, statistically. Especially if you leverage your Rituals and make sure you're fictionally positioned to roll with Advantage when you attack.

I see that, true. Thanks. I suppose if feels like this little critter would be a lot tougher than a level 1 PC which seems unthematic but also makes early adventuring nicely dangerous .