The Goo is almost certainly just unused building materials, a soup of nutrients and calories that the undeveloped monster cell contained in it uses for explosive growth, growing to size in seconds much like the formed monster’s do when evolving.
Seriously the reason the eggs turned into goliaths when forced to hatch quickly is because goliath is the simplest monster to develop, no electricity field generating organs, no teleportation manipulation cells, just muscle, bone, common organs, and a relatively simple brain structure. Even fire breath only requires slight modification of the digestive system.
To grow a wraith, kraken, behemoth, or other monster form requires a more complex time for the base cell to grow and develop it’s secondary base organelles, the one’s that allow them the vast and differentiated abilities exhibited by the various forms, after which they grow and break forth on their own.
Not official, my head cannon, and food for thought only