From what everyone was saying, I got the impression that we can challenge ALL of the special BDSP characters in the Battle Tower (in both Singles and Doubles).

But, according to Serebii, you can only challenge the 8 Gym Leaders and Cynthia in Singles, whereas the Team Galactic members, the Elite 4, the Stat Trainer's and Barry, can only be battled in Doubles.

Furthermore, only 2 of the Elite 4 members appear in the Battle Tower (Bertha and Flint) while Lucian and Aaron do not appear in the Battle Tower at all.

Could someone please confirm if Serebii is correct, or is this some kind of error on the page?

Here is the Battle Tower page:
https://serebii.net/brilliantdiamondshiningpearl/battletower.shtml

I generally prefer to play Single Battles and I was hoping to meet all of the special NPCs there. It would kinda suck if most of them are locked behind Doubles :(