Is lord Shiva himself of tamasic guna, i.e: he himself has tamasic guna, or he presides only over tamasic guna of people?.
He (Shiva) was beyond from three gunas as per many scriptures. A reference from vedasara Shiva stavah by Aadiguru shankaracharya.
गिरीशं गणेशं गले नीलवर्णम गवेन्द्रादिरूढ़म् गुणातीतरुपम् । भवं भास्वरं भस्मनाभूषितांगम् , भवानी कलत्रं भजे पंचवक्त्रम्।। Girisham ganesham galae neelvarnam, gavendradiroodam gunatitrupam । bhavam bhaswaram bhasmana bhushitangam, bhavani kalaktram bhaje panchavaktram ।।
So here gunatitrupam means the one who was beyond from three gunas or qualities. It was another thing that his physical appearance looks like tamo guna pradhana.
Based on what Brahmaji said in Rudra Samhita of the Shiva Purana itself which claims to be the greatest of all Puranas :- Vishnu is stated as only a demi-god on the same level as Rudra. While Shiva is free from all Gunas. And Vishnu is not only on the same level as Rudra, he is called Tamasic. Also Laxmi is called Tamasic.
Vishnu, Rudra and I (Brahma) represent the Gunas.
Siva is free from Gunas. He is the Supreme Brahman, the undecaying.
Vishnu is of Sattva attribute, I (Brahma) am of Rajas attribute and Rudra is of Tamas attribute. This is only in the view of activities in this world. But in fact in name it is otherwise.
Vishnu is Taamasik nature within
Goddess of speech is Rajasic nature; Sati is of Saattvika nature and Lakshmi is of Taamasika nature; the great Goddess Sivaa is of three natures.
So only Rudra presides over Tamas and Vishnu is actually Tamasic according to this.