rotafix alone if done properly should be enough. I am not a lawyer, this is not advice, please don't sue me when you die. Red locktite rarely fails if you put the cog on real tight. Either or, but I would say both is overkill. A properly rotafixed cog can take two people with all their might to take off.

If you can run a lockring, do it if you value safety, simple as that. Some freewheel hubs take BB lockrings in case you didn't know that.