Notes
The only command is /pltp, which can be disabled in the settings file. When you use it it teleports you to the Planetoid world, at the spawn location. I suggest disabling it and using other multiworld plugins to do the actual teleporting and moving between worlds
You do not need to add anything to your bukkit.yml to use the Planetoids world. Planetoids creates its world during startup, after the default world is loaded. HOWEVER, if you ARE using a multiworld plugin, and would like to use Planetoids as your chunk generator, simply set 'autocreateworld' to 'false' in the Planetoids config, and follow your plugin's directions for using a custom generator. If the plugin needs the exact class name for the generator, it is 'org.canis85.planetoidgen.PGChunkGenerator'.
Regarding Density: If you're planning to use a teleport/flying plugin to hop from planet to planet, you'll need a lower density so that the generator can keep up (<1000). If you plan on restricting your players and making them build their way from planet to planet, you'll want a higher density (>5000) so they don't have to build so far to get to the next planetoid (this is assuming default planetoid sizes). In general, the higher the density, the harder the generator has to work to create all your Planetoids
Spawn point is on a hard-coded planetoid that generates with a leaf shell and a log core. So even if you don't have leaf/log in your settings file, the spawn planet will always be this, so that players have something to start building with (in testing I started on obsidian planets sometimes...)
If you changed the config but you're still getting (or not getting) shell/core types that you've removed/added, remember that you have to move into a new "system" (or delete the files in /plugins/Planetoids/Systems and delete the world) to see the new types. Each "system" is 50x50 chunks, so you should see new planetoids at x or z = +/-800 (and 1600, and so on)
Make sure you leave at least one core type that doesn't generate heat (burning_furnace, fire, glowstone, jack_o_lantern, stationary_lava, lava). The generator has some internal logic to help prevent stuff like a wood shell with a lava core, and the generator code will freeze if it can't find an alternate core
The new Beta release that supports custom data values relies on an internal BlockPopulator class. If you set your own custom BlockPopulator for your Planetoids world, your custom data values won't work.