The affix ṅīṣ is always used in the chandas (Vedas) in forming the feminine of the words bahu etc.,
Thus बह्वीषु हित्वा प्रपिवन् ॥ Here बह्वी is the name of a herb.
The word नित्य 'always' is used in the aphorism, more for the sake of the subsequent sutra, which it governs than for this sutra. For the word 'optionally' does not govern this, and the aphorism even without the word 'nitya'? would have been a necessary rule and not optional: for आरम्भसामर्थ्यादेव नित्योविधिः सिद्धः, योगारम्भश्चिन्त्य योजनः ॥,