As we have discussed before on another thread, there is some descrepency on his birthdate. It seems to have been 1882, since Felix said there was a five year age difference and he was born in 1887 for sure. Also that Zenaida was worried about the duel because in every generation only one heir had lived past 25 and he was near his 26th bday in 1908 making him again born in 1882. I have thought maybe he was already on the way when the parents were married, and this is why his birthdate was hushed or confused. This was very common in the olden times when 'out of wedlock' babies or premarital sex was scandelous. This would also account for Zenaida's rushed and small wedding (in April 1882) though she was the richest and most eligible girl in Russia!
Now on the name. Felix's father was Felix Sumarakov-Elston. When he and Zenaida married, Zenaida's father petitioned the Tsar to allow the husband and future children to take the Yussoupov name since Zenaida was the last of her line and the name would die out. This was not granted until 1891 when he died, so Felix and his brother were born with the Sumarakov-Elston name before they were allowed to be called Prince Yussoupov. Sometimes they carry both titles, depending on the book you read.