The translation is correct. (Just in case, にしています means (we) have made it a rule that..)
Regarding only until 6 o'clock, it follows the basic rule of 'reversing'. That is, Japanese uses post-positions which reverses the English prepositions. So までだけ is the correct one. (If until only works in English, forget the explanation.)
Nothing should be technically wrong about 6時までだけにしています, but I feel 6 o'clock may be rather too usual to use only. Possibly it is early for owls, in which case using までだけ is fair.