Welcome to you and Nacho to YT. First off you just never know what size a Yorkie will end up when fully grown and mature. I've seen some at just over one pound at 14 weeks, end up at over 6 pounds and I've see 3 pound ones at 12 weeks end up at only five pounds. It's very difficult to know the outcome.
As for harness verses collar, I would use a harness for walks since the small dogs have such a fragil trachea and pressure on it could cause damage or collapse.
Hope we get to meet Nacho real soon.
