Like you said, for Faerie, many of their closely-related monster-types also apply to other monsters in game. Faeries as they appear in Ryslig are also heavily modified from those you find in most mythologies — the insect aspects were added in-game to give them a more traditionally "monstrous" appearance. Unfortunately, history is not rife with very many insect-based monsters (that aren't giant spiders). As such, it's difficult for us to provide you with an extant list of what would or wouldn't be allowed.

For a Faerie in this case, your "closely related" options swing both towards the fae and to insects, or to an amalgam of each. In a circumstance like you mentioned, where for instance a Dryad straddles the line between a Faerie and a Nymph, we would allow both Ryslig monsters to access this shared mythology, so long as they do not use it to stray completely outside their own monster-type. Fairies in myth are often associated with elements, with household or natural spirits, and in some branches of Christian myth faeries are associated with the hierarchy of angels. With review and approval, you would be allowed to use any of these things to construct an alternate monster form.

We apologize for how long it's taken to get back to you, but we hope this information is helpful!