An obstacle in Shenzhen (less than elsewhere in Guangdong province) is that many people's first language is Cantonese, not Mandarin. Although the fact that Shenzhen is such a melting pot makes it reasonably easy to find people with clean accents for language exchange. If you have any specific questions regarding the city feel from to PM me, I'll be in SZ until early August.
Where to then Icebear?
Dialects make things very tricky anywhere, so you need to check which people do ahve a relatively standard Beijing accent.