According to Serebii, you cannot encounter all the special NPCs in both Singles and Doubles in the Battle Tower. Instead, in Singles you can only encounter the 8 Gym Leaders, Cynthia and Palmer. All the other NPCs are exclusive to Double Battles (the Elite 4, the Stat Trainers and the Team Galactic members).

Can someone PLEASE confirm if this is true, or is it fake news? Because I personally prefer Single Battles, and I was hoping to encounter all of the Special NPCs there.