वेदे उषस्-शब्दः

Acharya Sayana treated उषस् as Feminine Gender wherever it refers to देवता. For example R.V.10-8-4, where he explains as उष उषः=सर्वस्याः एव उषसः and R.V.1-141-7, where he explains as नक्तोषासा=(उक्तगुणविशिष्टे) अहोरात्रदेवते (अभिमानिदेवताद्वन्द्वे पूर्वपरयोः उषासा इति आदेशः). Where उषस् is used to refer “time” there Sayana treated that as Masculine or neuter genders. For example R.V.6-28-1, and 1-151-5.

Hymn you mentioned is सखाभूदश्विनोरुषाः (R.V.4-52-2). Here also Sayana treats उषस् as feminine gender and explains as सखा=समानख्याना=सह स्तूयमाना. Another instance you quoted is देवीमुषसं स्वरावहन्तीम् (R.V.5-80-1) where Sayana treats उषस् as feminine gender and he explains as देवीं=द्योतमानाम्, उषसं, स्वरावहन्तीं=सूर्यं गच्छन्तीम् । One more instance you quoted is उषासमिन्द्र संपिणक् (R.V.4-30-9). Here also Sayana treats as feminine gender and explains as उषासम् (उषसम्)=उषोदेवीम् ।

In Veda any word can be in any gender and it can 

be in any form. As said 

सुप्-तिङ्-उपग्रह(उपसर्गः)-लिङ्ग​-नराणां काल​-हल्-अच्-स्वर​-कर्तृ-यङां 

च​ । 

व्यत्ययम् इच्छति शास्त्रकृत् एषाम् सः अपि च सिध्यति बाहुलकेन

॥ (वार्त्तिकम् ३-१-८५). 

However, Sayana treats उषस् as feminine gender wherever it refers to देवता. In other places it is either in masculine gender or in neuter gender.

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