I kept a corn and 2 garters together for 2 years and had no problems. One garter was just a little smaller than the corn and the other was a baby. They always shared the same hide (even though there were several) and curled up together.

Eventually I had to move and I had to downsize my collection to do so and I sold the 2 garters that lived with the corn and the corn acted very odd for quite some time after. It was clear that she missed her room mates.

BUT there is no doubt a risk, just as there is when you house any 2 snakes together, and this risk may be slightly higher for the same reasons that Stefan-A has already mentioned.