Is a self realised shudra an exception to varna ashram dharma and can become guru? Or can he simply become mentor? And can he give any form of diksha to disciple?
As Pradip sir mentioned, anyone who knows Spiritual truth can bestow Knowledge. Ramanujacharya accepted Kanchipurna a Sudra as Guru. Nammalvar another Sudra wrote Divya Prabandham which is known as Tamil Veda. Dharmavyadha and Vidura were Sudras who bestowed knowledge even to Bramhins and Kings. These quotes proove that even a Sudra can be a Guru-
Yajnavalkya says By obtaining Brahmajnana from a Brahmana or Kshatriya or Vaishya or Shudra or even a person of low caste, person with faith should constantly practice the same.(Mahabharata Shanti Parva 378,Shl 87)
Whether one is a Brahmana, a Sannyasi or a Sudra , regardless of what he is , he can become a spiritual master if he knows the science of Kṛṣṇa.(CC Madhya 8.128)