Yes necessary to be a Singh, a Khalsa warrior but not a requirement to be a good or even an enlightened Sikh. I have studied "Brahmin philosophy" to some degree, I think their views are largely misrepresented by Sikh missionaries. Brahmins have lived well with Gurus. Three of them went with Guru Tegh Bahadur Sahib to their martyrdom, where they were tortured in front of Guru Sahib. Many of the contributions to Guru Granth Sahib come from Brahmins. I think there's more than 10 Brahmins whose bani is in Guru Granth Sahib. Nirmalas and Brahmins both devote their life to study. Kabir says "Brahmin is one who studies Brahm (God)." I don't see why someone who studies cannot be a Sikh.