Q. That's the guy I talked to/was talking to. I know "I talked to" is fine, but would "I was talking to" also be natural to use here?
Can I explain this by using some examples
That's the guy I was talking to, who was picking his nose, all the time during our conversation.
In this instance you would not say talked to. Although the conversation was in the past you are discussing actions that were taking place at that time.
In your example That's him. That's the guy I talked to (yesterday) would be more natural. Whilst That's him. That's the guy I was talking to about ..blab blab