The stench is mostly contained inside Trubbish and Garbador's bodies unless it's attacking and stuff, from what I know about reading the pokedex' entries.

But yeah, Weezing and Muk wouldn't make for good roommates.

As for Magmar, I think that's just it's body temperature...which y'know, is contained in its body. Sure the area would be considerably warmer, but I'd like to think of a Magmar as my personal portable heater~