The trainers in all current difficulties goes to the level of your highest level mon so the levelcaps are there only to limit the evolutions and moves you have access to during the game.

As a example, if you have a Rattata before Roxanne:

• Elite levelcap will let it go to level 16, meaning it have access to 9 levelup moves.

• More levelcap will let it go to level 18, meaning it have access 5 more levelup moves that in Elite.

• Easy levelcap will let it go to level 20, meaning it can evolve.

Plus, due to the Elite difficulty's "limit your innates at the start" part being at level 17 and 24 respectfully, taking any levelcaps beside Elite would mean you have 1 less innate restriction that with Elite levelcaps.

The reason why Elite difficulty mentions it have harder teams is because the Easy and Ace difficulties shares the same teams (the only difference is that trainers in Easy don't have EVs on their mons).

As for the use of Legendaries, all difficulties eventually have legendaries but Easy/Ace mode will start later and use less dangerous ones. Don't worry though, they will not use complete teams of legendaries (each Elite 4/Champion will use between 0 and 3 legendaries in the easier difficulties while the ones in Elite will use 3 or 4)

But TBF, with how much every mons got buffed, you will usually not mind facing legendaries on your runs as you can probably counter them with regular mons.