slugs which are locomoting and fighting me seem to react ok to stones being thrown.
slugs in guard mode, before I disturb them, act odd - I throw something and it hits them immediately. Bizarre. Until they have left that square they start off in things will still hit them instantly when thrown. No idea why, I will have to investigate.
I'm assuming that capsule stats in monster def relate to the collision capsule - i checked the domentions in 3dsmax against the snail skeleton and it all seems reasonable - I will keep tinkering for a bit.
<edit> I checked the binary data of the slug's capsule node against the snail from the asset pack and it matches exactly, so I am not sure yet what the fix will be.