I see debate, but no definitive answer. Here is one and how we are going to work with this:
You want to make a decision involving two species based on their nature? Then both FMs have to agree. If they don't, you don't play ball. It's as simple as that.
All the cards are not just in Frostjaeger's camp on this, though.
@Wes created the Elysians: he gets to weight in for the Archangels and Plebians. That's his call as creator of those species, regardless of in whom's FM's hands they are in. I've seen Elysian FMs come and go, but the species' creator is still here on the site and he's active, so, we will defer to him.
However, Zakalwe - one of the earlier Elysian FMs - created the Caelisolans.
That is under Elysian FM purview.
That, Frostjaeger does have final word on. So, if it's between Edto and Frostjaeger deciding if the Caelisolan body has compatibility, then the current Elysian FM should be able to veto this.
There would be one exception to this, but it's only regarding antagonistic species. For example: Mishhu tentacle stingers paralyze, nekos still drop like flies when stung despite supposed resistance/immunities to toxins. When a Game Master is employing a creature as advertised and leapfrog the "go through the FM" process. That can be sorted out later, treated as an exception, or handwaved as artifice.
This exception does not apply to a playable species, which the Vekimen are.