Interacting through a baby gate/exercise pen–realize that these are NOT secure. Dogs can knock them over or jump over them.
That doesn’t mean they can’t be great tools when used under supervision, though. If you know your dog physically can’t make it over the barrier or if they are well-trained you may not need to have them on leash. The unknown/reactive dog should be on leash and it should be treated as training time.
In the case of dog-selective dogs that might not appreciate an overly enthusiastic friend stepping on them, an exercise pen can provide a physical barrier but still have all dogs feeling comfortable and part of the family.
You may not have to have the dogs on leash but you should be in the same room, supervising them. Exercise pens (even when hooked into the wall) can easily be knocked over and are not considered a secure barrier.