So assuming Aelita and Ren are no longer mono-type trainers.
In the end of episode 10 Aelita says that she will claim the position of Sensei possibly also becoming the Fighting type Gym Leader.She says that she will go on a journey to find the springs and the only mon she will take with her is her Hawlucha.So i think it makes sense for her to start not being a mono-type trainer and eventually becoming one.
As for Ren, i dont think it suits him that much.The reason Ren left Goldenleaf town was to train so that he can beat Narcissa and change his people.Dark type mons resist ghost type and at the same time are super effective to them.So i think it would make more sense that he is a dark-type mono trainer until he defeats Narcissa.Or unlti he decides to join Team Xen.You know " New me, new team "
The only reason i can think of as to why it would suit him not being a dark-type mono trainer is the fact that he has challenged Narcissa many time in the past and was unsuccessful and it would make more sense for him to lose if he wasnt a dark-type mono trainer.