Has it ever been legal to plant a prisoner in a jail cell with a suspect to get a confession from the suspect?

No. Anything the government does to intentionally elicit a confession, or any other information for that matter, is illegal once the 6th amendment right to counsel comes in if and only if the person can be classified as an agent of the government.  Now 'agent' of the government does not mean police officer, does not mean FBI, it means anyone doing what the government tells them to do for a particular purpose.  Hope I helped, good luck!