Phoenix

Hello, I am new to this forum and, since I am studying Sanskrit on my own in preparation for grad school, am overjoyed to find this community! My question is: according to which rule of vowel gradation and/or internal sandhi does "Siva" become "Saiva" and "VisnuVaisnava"? Thanks for your help! Phoenix P.S. Anyone else using Madhav Deshpande's "Samskrtasubodhini" (U of Mich. P) as a primary study text?