Prankster Meowstic

Keep them low level, give the player the xp share earlier.

Yeah, personally I think the AI simply isn't close to good enough to use Trick Room correctly.

I generally find the double battles to be easier because the AI is so derpy with them. If you have a poke that has protect or detect in its learned moveset they are all pretty much a breeze. The psychic gym leader was probably the easiest because setting up trick room is a terrible idea when the opponent has an Escavalier on the loose. It is probably different for players with switch on. I found Corey to be the hardest just because of the auto damage from the field followed by Terra because of the monster that is Garchomp.

Gamefreak gave Flygon the axe when they came up with Garchomp.

I think you could replace Meowstic in the early game with a less used psychic type. Prankster is one of the most broken abilities in the game (especially in double battles) and is actually really useful in game as well because the AI has no idea how to deal with it. Also, it's learned moveset makes it a superior option imo to Sableye which is obviously the better poke competively. Getting up screens with priority in gym battles as been at least as useful as having Infernape as the starter.

I've been playing the game for a couple of months and I'd say the biggest limiting factor preventing variety of team building is not necessarily what pokemon are available but what moves are available. Since there very few good TMs available and the move tutors are all really late, it boxes in the player into picking pokemon which learn good moves via level up. This makes using a pokemon like Kabutops essentially impossible even if it would be available earlier in the game and makes pokemon like Flygon, Arcanine or Male Meowstic highly desirable.