Addressing an acquaintance as 小妹 I met a woman four or so years younger to me at a conference (not work related). After getting to know her slightly more I later sent a text agreeing to meet tomorrow with "好吧 明天见 小妹". We briefly discussed our age difference prior for some reason. 

My intention was to be playful and friendly with "younger sister". Is 小妹 rude and / or completely unfitting in this situation?
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)