Reaction to First reply:
It's possibile to script that but it would make it a bit harder as there may be 1000 biomes spawning guardians so I'd need to save each biome indiviually, much like the chest script saves each chest location placed by players ( Note, no worries about server load issues as a regular server can take up to 10.000 task commands a tick ( 20 ticks is one second )
It would make it a whole lot easier to calculate on percentage. here's a explaination:
Within terraincontrol you can set the maximum amount of guardians to spawn. Let's say you have this set at 30 guardians, then replacing 10% of these guardians would result in having 3 elder guardians at a time
Reaction to Second reply:
It's pretty easy to create the biome actually, simply copy over the existing ocean biome, rename it and use it as isle within the existing biome. I'm not sure what you meant by tagging the elder guardians and guardians aswell as manually placing it in, You're using something else then terraincontrol for this? like a schematic?
Then about your question, if you setup a border, make your world in terraincontrol and remove the border it will use the new biomeconfigs to generate the lands behind the border. I'd have to warn that the transition may be off if you alter the biomeconfigs and esspecially when you change the seed you've used.
Anyway, take your time and experiment a bit before implementing stuff pernamently it will help you alot in a further stage
( Sorry if some things seemed unclear, I'm currently doing server management for a Ancient Realms Gameserver and my mind is completely set to that task I will soon return back to doing terraincontrol stuff though )