It's entirely possible that the Gorn Captain fighting Kirk in TOS was nearing the end of its lifecycle.
Its slow and plodding demeanor may just be how an elderly Gorn behaves.
The Metrons didn't give either side the chance to select their best fighter, they just chose the Captain of each starship.
Perhaps Gorn Commanders are picked for their lifetime of knowledge, not their fighting skills.
Most lizards here on Earth only live to be 30 or 40 years old, maybe the Gorn don't live much beyond that either.
That's stretching it. If the adult gorns look like xenomorphs its a definite canon violation. I don't expect them to look like rubber suits but I would like to a upright lizard man.