Downsizing a beacon moth (Huge) to Small yields...
Str -3, Dex 25, Con 4, Int 2, Wis 12, Cha 16
OK, so maybe that doesn't work so well!
Let's try a gloomwing (L) to Small...
Str -1, Dex 20, Con 7, Int 2, Wis 12, Cha 10
OK, still no good.
How about increasing a giant fly (T) to Small...
Str 5, Dex 17, Con 10, Int --, Wis 10, Cha 2
That seems a bit more on-track. With a Dex modifier of +3, and a size modifier of +1, that translates its AC perfectly. Assuming Cha-based for the deathglow effect, Cha needs to be at least decent.
Thoughts?