I've introduced younger chicks in with others that were five weeks older but only ever in groups of at least 5. They also have a lot of room, multiple feeders/waterers, and multiple heat sources. But I use heat plates so it would be different if using a heat lamp.
It's going to depend on the temperament of RIR and space/resources. It's also going to be a but tricky adding a lone chick because it's going to automatically be the outsider. If possible I suggest getting more chicks. The more there are, the less likely one gets singled out and it's good for the new ones to have buddies. You'll just have to put them together and see what happens. There's really no way to know until you try.