Go with your gut.
My dogs have always acted differently with dogs in the home versus dogs in a strange environment. Sometimes for the better, sometimes for the worse, but always different.
It may not be "love at first sight" type of a deal, but it also doesn't sound like a horrible match.
So go with your gut.
See the thing is, I don't know what my gut is telling me...Astro's reaction threw me for such a loop. Going to order dinner and think it over.
While they were actually playing did Astro appear to enjoy himself? Did they ever get over aroused and have to be stopped? Did they take turns as far as who chased who? If the Boxer ran away or paused did Astro move towards him to continue playing?
You have said that you want an adolescent who will frequently play with Astro. So it's very likely, unless it's the sort of shelter/rescue that has frequent play groups with the dogs, that Astro is going to have to teach the young dog manners no matter who you get. So if the actual play, while it was happening, appeared to be enjoyed by both, and the Boxer did back off when Astro corrected him, AND Astro's correction was appropriate (both in what triggered it and in it's severity) then I wouldn't be super concerned. That said it's sooooo hard to say when you've only got one meeting to go off of. Can they meet again tomorrow? I'd also try to see how easily you can distract and redirect the Boxer from Astro, that will help you see how easy/difficult stopping the pestering will be once you're home. Realize you likely will have to help Astro out in teaching the boxer when to leave Astro alone.
The Boxer sounds fairly typical of the breed though, their play style can be alarming to other dogs so even normally friendly dogs don't always get along with them. Add to that the fact that the boxer may be under exercised and under stimulated, plus Astro was in an unfamiliar place in hot weather, it just was a lot for Astro to deal with.
Again, really hard to say. It depends just how annoying and persistent the boxer is and if Astro will be more or less tolerant at home.
Also, I checked over the post but couldn't find an answer, is the boxer a male or female?
They played very happily for the most part, but actually now that you've mentioned it I think the tension started earlier than I first thought - I think the lead up was the boxer bowling Astro over a couple times when they were playing in the yard. They took turns and chased reciprocally, but as Astro got tired it turned into the boxer following more often, and Astro looking for an escape route.
It was hard to judge even for the staff. Astro did initiate play again a few times indoors but it was so half-hearted, more like he was just giving in to the general tone of "this other galumph wants to play." I felt his corrections were a little unnecessarily harsh, they weren't violent but they were, in the words of the behaviorist, quite pushy. She said she didn't want to use the word dominant for its problematic/complex connotations, but honestly Astro did look a bit dominant to us. There were a few times where Astro marched up to the boxer when he was laying down trying to initiate play (in that kind of tense springy down) and stood over him staring directly at him like "you WILL knock that off," like he really meant business if the boxer didn't acquiesce. Honestly both were being so 100000% not-subtle but it was still hard to suss out somehow? The boxer seemed tractable but I think if he hadn't been, things might have escalated...I could see them being great together once the boxer learned to let up, but I could also see Astro over correcting him for doing what Astro himself always does! Apparently he can dish it but he can't take it
I definitely would have to help teach the boxer to leave when Astro is done. Was hard to gauge when I have no relationship to the boxer, and the dog has no training to speak of, not even name awareness.
The boxer is male, which is also something I'm thinking about...Astro has been great with both genders thus far, but this interaction makes me wonder. The boxer was being so appeasing (in a boxer-y way - he was still gunning for play) and Astro was peeved nevertheless. I don't know when the boxer was neutered exactly but it was recently. Astro was a pediatric neuter, he was done by the time I adopted him as a baby puppy.
I can't let them meet again tomorrow unfortunately.