So what does compromise look like?
My attempt was fairly straightforward. Get the cultural feel and mannerisms of a tortle, the physical benefits of a tortle's shell. The character's species is still from the curated list but has the feel and characteristics of a tortle. People might think they're a bit odd. Heck, I think it's a bit odd and I'm going to have to put some thought into what it means and how to handle it. But there was no need for me to add a new species.
Now, I'm not saying it's a perfect solution, but that's where conversations and compromises start. It doesn't start with "I must get everything I want 100%".
What else could a compromise look like if that's not it?