On our servers due to Mystcraft and RF Tools dimensions being really unfriendly to performance we have opted to disable world creation with these mods and enable our own custom setup.
We are currently using the Multiverse plugin (1.7.10 Servers) and the Nucleus plugin (1.10.2 servers) to create personal worlds for players that want them.
In order to get a personal world you must use the reward store on the website and purchase it with your website points.
There is a selection of different world sizes for you to choose from.
Basically what this means for ex. personal world "500". The 500 is the world border radius of that world.
So when its created this is set and you should have a circular (1.7.10 servers) or square (1.10.2 servers) border on your world with a radius of 500 from the 0,0 coordinates
Also available in the reward store is the ability to disable the weather and daylight cycle (1.7.10 only) in your world.
This can also be added at the time of purchase as well in the options.
/myworld - Teleports you to the spawnpoint of your personal world
/addtomyworld (Name) - Add the specified player to have permissions to build in your world
/removefrommyworld (Name) - Remove the specified player from build permissions to your world
/world tp yourname_worldname - Teleports you to the spawnpoint of your personal world
Note: With personal worlds its possible for any biome to spawn in it (Including biomes from Twilight Forest and other mods)
Also Note: Ensure that you still claim your base (Though not as critical as the overworld) as other players you tp to your world may still be able to access it. Additionally bases that are inactive for 30 days are still subject to removal.