Yes, you can teach your rattie to walk on a leash. Make sure that the harness fits the rattie properly and use a short leash to start. Train the rattie inside just in case the rattie would "slip the harness". Never, I repeat, never use a collar on a rattie. For a collar to be tight enough for a rattie not to get out of... it would do serious harm to the rattie, if not kill it.
In the high plains of South Dakota, USA