For a beginner or someone who has difficulties to read fluently what is the minimum to say during the selichos ?
There’s no required amount of Selichot. Whatever you can say is good.
Is it true that the Rabbi said that one should not say tahanon at the end of the slichot?
In the Rav’s shul we only say Tachanun once. So yes we skip the one by selichos.
Did he skip other parts of the standard Sephardic Selichot or only the Viduy?
The Rabbi used to join the Ashkenazi minyan in his Shul amd said what they said.
Not sure if this was brought up in other posts but the Rabbi says to skip מכניסי רחמים. No Davening to Malachim.