Does the same rule apply if the gathering has some women with hijab and some without hijab?

Yes any gathering which has been organised just for amusement and not for any religious or educational purpose is not good and we should avoid to participate in it. Because our participation in such gathering is the sign that not only we are not against this type of gathering but we are one of the supporter of such gathering. Therefore better to avoid such mixed gathering where there are non Hijab woman are participation and it’s aim is nothing but just to have some fun or amusement but not for educational purpose.