Scaling a stirge gives 125-225 lb, roughly.
Works for me.
Right, once Shade plugs that weight in it just needs some tactics and a bit more background text.
Not sure about the Gnat Swarm. Its AC seems rather high. Maybe we'd be getter off making them "giant" gnats (Diminutive Vermin) and maybe lowering the Dex by 2 or 4, giving them Armour Class 18 to 16.
A high Dex and AC does seem appropriate for flies though, and it saves us having to rewrite masquerade to allow for a swarm of non-Fine sized insects. Although we might as well consider the possibility
Speaking of which, the original monster could only shrink to the size of an ordinary insect, suggesting the Change Size should be:Change Size (Su): As a standard action, a demonic sawfly can magically shrink to the size of an ordinary insect. This functions like a reduce person spell, except that the ability can work on the demonic sawfly and it reduces its size to Fine. A demonic sawfly can return to normal size as a swift action.
Alternatively, we could make the Swarm distraction only, but IIRC the AD&D insect swarm spell inflicted damage.
Flavor text and tactics look good. Updated.
I'd prefer Fine gnats.
I suppose we need to decide, then. I'm perfectly content with just Fine size, and thus using the revised Change Size (but current Masquerade).
I like sticking with damage, but I'm not sure it needs wounding. Swarms of gnats don't generally leave me in a pool of my own blood!![]()
Swarms of gnats don't normally kill an average Commoner in a few rounds either.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.