Axolotls and Glow Squid appear to spawn EXTREMELY infrequently, to the point where I'm almost certain it's a bug. Even in a huge, flooded cave absolutely none spawned. I had to search many of these flooded caves before I found a handful, so I know they do spawn. But if their spawnrates are meant to be akin to other passive mobs such as cows and pigs, their spawnrate is basically negligible.