As far as I remember, it is also possible to choose the commutation relations for different fermions. However, traditionally, anti-commutation relations are chosen for creation and annihilation operators of different fermions and commutation relations for creation and annihilation operators of fermions and bosons or different bosons.

As far as I remember, it is also possible to choose the commutation relations for different fermions. However, traditionally, anti-commutation relations are chosen for creation and annihilation operators of different fermions and commutation relations for creation and annihilation operators of fermions and bosons or different bosons. EDIT:(12/1/2016) See, e.g., https://arxiv.org/abs/1312.0831