You should be able to meet people in the park, that's for sure. I wish I could recall exactly what time I was there, but there were several groups. The Chinese view partner dancing as another good form of gentle exercise, like Tai Chi, that should be done daily for health benefits. I have a friend who's mother lives in Hong Kong, and she gets up very early every day to go ballroom dance in the park near her home.
Perhaps someone there could point you to a dance club, too.
Even if you don't find the dancers, the Temple of Heaven is wonderful and the park is delightful. Aside from dancers, you'll see groups of people who gather to play traditional instruments, or badminton, or mah johng. It's really a place where people go to relax and have fun outdoors.
Do you speak any Chinese? I knew a TINY bit when I was there, and it helped a lot.
Xi'an is beautiful, I really liked it there too. I didn't see any dancers but I was only there for three days.