Is it true that Eric the clown was played by Jon Favreau from Swingers? I heard that, but don't believe it. Was it before Swingers?

Same guy.
The Seinfeld part was before Swingers.

Eric the Clown was in
084 - The Fire
pc: 518, season 5, episode 20
Broadcast date: May 5, 1994

Jon Favreau also wrote Swingers (http://us.imdb.com/title/tt0117802/) (1996)

Thanks for the info. I have to see that episode again and check it out.

I would have liked to see Eric the Clown and Kramer get together.