I would scrape or brush off the cog rust where the chain will mesh on the teeth. I wouldn't spend much time doing it, just enough so that the chain rubs off less remaining rust. Either way I would soon remove the chain and give it a thorough cleaning to flush away the rust filled lube.

Interesting story- Years ago I had a 20" 3 speed folding bike. I had loaned it to a friend a year before and got it back with a very rusty chain, almost enough to be not useable. But I was able to free up all the links so reinstalled it then re tensioned the chain. Lubed then rode the bike work a few times. After a couple of weeks I noticed the chain was quite loose. The wheel was in the same location in the drop out as before. But I also noticed that the chain was much more freely turning too. My theory was that the rust that coated the chain link interiors got scraped/rubbed off then carried away as a slurry of oil and rust dust. As the rust left the chain link interior the roller and other parts develop more play and the chain "grows" in length. When I cleaned the chain the solvent run off was very brown. The, now cleaned, chain felt loose and free in the links. Reinstalled it, again re tensioned the chain, lube and ride. The chain proved to be stable in it's now far more rust free state. The bike ran smoothly and the chain remained well tensioned. Andy.