The thing is - we just don't know very much about Tessio. In the novel he doesn't even have a first name. Clemenza is smart in the way he plans the revenge on the two young guys who hurt Bonasera's daughter and the killing of Paulie.
But I've always had the feeling that some of his actions were a little risky.
- When they steal the carpet, he's ready to shoot a police officer. For a carpet!
- When he tricks Paulie, he goes for lunch. This could have gone wrong.
- Telling Rocco to leave the gun wasn't smart because of fingerprints, unless of course, he had prepared it with the magic tape.

For the "box & chain thing": http://en.wikipedia.org/wiki/Flush_toilet#History