I was interested in this yankii and chiimaa thing too!
I did some research and didn't find too much information since it seems to be a very Japanese thing.
From what I gather yankii are like the young punks of Japan. They are rebellious and dress in a punk style. Kind of like motorcycle gang fashion. It seems to be a cultural thing that has been around for generations. Mostly popular in the 1980's and 1990's.
Chiimaa seem to be the same thing but is a bit newer. Kind of like the new yankii. That's why Gackt was asking Ayu what was around during her time and she said chiimaa. Gackt being older than Ayu said he related to yankii. But I just can't seem to find any info on chiimaa.
That's my understanding.
http://www.japantimes.co.jp/life/200...-your-yankees/