If I remember correctly Surly sizing is large compared to the "big three" and lots of other high volume builders. So you'd need a Surly sized 1-2 sizes lower than you'd take on say a Trek.

So, if I'm remembering this correctly, then your shop is likely correct that the 51 Trek will be good enough for a parking lot test, depending on what you are trying to feel.

You may get an idea of what slacker touring geometry is going to feel like, but if it is indeed too big you aren't going to get a 100% accurate idea of ride characteristics and handling because that does change with your proportions relative to the bike.