How to choose a second dog?

I want to get an adult one from the shelter, anything from 1 yr to older. I already have picked out a few that fit our lifestyle ( active and cat and dog friendly), and I have the option to take my dog there. But won't making them meet like that distress either or both dogs? Mine likes dogs but needs time to... show more I want to get an adult one from the shelter, anything from 1 yr to older. I already have picked out a few that fit our lifestyle ( active and cat and dog friendly), and I have the option to take my dog there.
But won't making them meet like that distress either or both dogs? Mine likes dogs but needs time to warm up to them. If they're barking he'd try to get out of his collar and run away, if they're calm they're free to sniff him as they please.
They have a dog park there as well, would letting them loose together be a good idea? I understand that being leashed makes some dogs more tense.
Maybe since these dogs are dog friendly themselves, things will be easier. But I keep imagining them suddenly fighting in the car. So far, all the ones I like are males, and mine is too.

You could ignore all this and just tell me how it went when you picked out a second dog. That'd be even more useful.
Thanks for taking the time to read and answer this.
Update: * My dog is a year old and well trained.
Update 2: The thing is my dog is high energy himself, and I don't want a couch potato, I want to go out for long walks with both of them.
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