I and @Robijnvogel have been experimenting with multiworlds on a server with both Biomes O'Plenty and RTG installed, with unexpected results. Creation using non-default worldtypes when alternate world generators are present does not work as expected. This is a summary of what we have found thus far:

— The provider and worldtype options available for use are restricted by the level-type set in server.properties.

— If level-type in server.properties is DEFAULT, you can create only vanilla multiworlds, but can use any of the vanilla worldtypes DEFAULT, AMPLIFIED, LARGEBIOMES, FLAT. CUSTOMIZED does not seem to work on servers, even if the create command is run from a client. NORMAL-based worldtypes such as Extra Utilities XU_WALL can be created. BIOMESOP and RTG worldtypes cannot be created. Nether and End multiworlds also cannot be created; the only available provider is normal.

— If level-type in server.properties is RTG, some NORMAL worlds (FLAT for example) can be created, but AMPLIFIED appears to be unavailable. BIOMESOP also appears to be unavailable.

— In general, BIOMESOP multiworlds can be created only if server.properties level-type is also BIOMESOP. I haven't conclusively confirmed this yet but it LOOKS as though if server.properties level-type is BIOMESOP, ONLY BIOMESOP multiworlds can be created.

— It looks as though AMPLIFIED multiworlds cannot be created at all, even if server.properties sets level-type=AMPLIFIED.

As @Robijnvogel observed in #2344, it appears that server.properties level-type is overruling the provider/worldtype specified in the mw create command, whereas one would expect the reverse to be the case — mw create should override the level-type in server.properties.

Is this still an issue on 1.12.2 @UnixRonin ?

I don't know, I didn't know it had updated.

I'll try to find time to reinstall it and retest, but I can't make any promises about when I will get to it.