It would probably be best to ease your new dog into your household routine rather than letting him run free right off the bat. Dogs tend to be very territorial and may not appreciate the intrusion of an interloper, no matter how cute he may be. It might even be better to have your two pets meet each other in a neutral, public environment, where neither one has staked his claim. Doing so might make it easier for them to accept each other’s presence in your home.
Don’t expect your dogs to get friendly with each other overnight. Some dogs may need a “getting-to-know-you” period of a few days before they even approach either. Depending on how well your older dog accepts the new one, it might be necessary to have a few short acclimation “sessions” spaced out over several days before they will begin to get used to each other.