Shiva is Veda vedhaarthasaara, means the ultimate summary of Vedas is Shiva ! When He dictated the Shiv Purana to Sage Ved Vyas , then He is a Brahmin ! Shiva is the Guru to Parasuram, the avatar of Lord Vishnu , then Shiva is guru.
Bramha and Brahman are two different things : Lord Bramha is the creator of this Universe, where as Brahman is the Divine spirit, the formless God Himself, who is omnipresent. Vaishnavites call Lord Vishnu as the Brahman where as Shaivites call Lord Shiva as the Brahman.