They will NOT.

My first FFVI game was on the cart back in the good 'ol 90s, and my main party consisted of Shadow, Strago, Gogo, and Setzer. Shadow and Setzer had 9999 HP, but Gogo and Strago didn't, and they were all level 99.

So you MUST use the espers, unless this has been corrected int the PSX version.