There are two different types of twintails, as pointed out by mangatron here:
http://animangatron.blogspot.com/201...love-flow.html
Concerning whether I regard the girls you named as twintails:
Tousaka Rin: yes
Ichinose Kotomi: yes
Yui (Angel Beats): yes
Tsurugaya Otsuu: yes
Toudou Lilica: yes (as well as Ebert Lilica from Burn Up W)
Sunohara Mei: yes
Takamachi Nanoha: yes
Besides different twintail styles (behind the head or more on the sides), there are also different hair lenghts possible, also with shorter hair as on Sunohara Mei or Nanoha. Sometimes, not all the hair is used for the twintails (as with Lilica), resulting in a partial twintail.
We could start our own twintail girls catalog and then classify accordingly: back-bound/side-bound, long hair/short hair, full or partial twintail.