No, you can't eggify hostile mobs. The only reason why some managed to eggify an enraged guardian or enraged zombie is because it was originally a hostile mob but then got cured: turned back into a villager (neutral mob). And you can eggify those.
Or a old bug when nether hounds came out and were egg able for a bit but those were a custom coded mob that before were egg able because they where Neutral mob in vanilla minecraft.
So ... if I cure an enraged guardian then I can eggify it ? I want to figure out a way to get a guardian elder, because I saw a video online about how to make your building (out in the frontier) ungriefable, just put an elder guardian in the center of the building and when a griefer or somebody comes around it gives them mining fatigue which makes it almost impossible to mine blocks. I want to see if this is even possible. Making a building out of obsidian is still very griefable under normal conditions. Mojang needs to make a block that gives people mining fatigue when they come to within , say, 20 - 30 blocks of it. That would be SO COOL. And helpful for safe gameplay :>
An enraged guardian is a zombie and is a mob that Momentus spawns when to help him while you are fighting him. Elder Guardian is a boss mob in an underwater temple. Much different. But yes you can eggify an enraged guardian IF it spawned in as a zombie villager. Just need a golden apple and a bottle of weakening to do so.
Not on the Empire, no. That is: unless you build your stuff around an ocean monument, because that would be the only way to get a building near an elder guardian.