I only had 1 peachick at first so I got it a day old ameracauna that did really well. It got too big at 3 weeks and started jumping on the new little peachicks. So I had to put poor little Beardsley who thinks he's a peachick in with the chickens.
A cochin stays smaller so would make a good choice I think. I heard you shouldn't use older chicks that have had time to catch any diseases and not to leave them in for long but I'm just starting out. I also read that you shouldn't put turkeys in with other birds so I wouldn't use one of those (they are "special" and have to be taught to eat themselves so wouldn't make good trainers anyway). Not sure about ducks.