Can আপনি choose different trainer sprites for the trading room like in Diamond, Pearl, and Platinum?

not really, 1 diffrent 2 the same.

Yes আপনি can. It's in Voilet City, where the first gym is just like Oreburgh City in Sinnoh.
If আপনি mean: example: Blackbelt, Ace Trainer, Psychic, ect. But আপনি might not get what আপনি want, there's only four choices of the trainers.