Addressing an acquaintance as 小妹
Just call her 姑娘

If she does not have business relations with you, it is absolutely OK.

Sometimes 小妹 is romantic involved, so it is not fit formal speaking.

小妹 is not very popular nowadays since it can be vague sometimes, but many chinese words can be vague like 小姐(young woman, also means whore), but 小姐 is still used in business and formal.

Here is a list you can use from the very formal to very rude and cursing.

女士 (formal)

小姐 (careful, also means whore outside business environment)

姑娘 

妹子, 小妹, 老妹 (younger than you)

姐, 大姐 (older than you)

小妞 (rude, you don't want to be friends)

娘们, 小娘们, 老娘们(very rude, you want to have row)

破鞋 (dissipated and unconventional, you want to have a fight)

婊子(whore, you want to have a war)