Ninja FV

Heads up: magnemite isn't available until around 11th Gym, and both ralts and shellder come 2 Gyms later. They're still very useful to have once you get them, but you're definitely gonna have to use other pokes until you get there.

As for Flying Types, fletchling, fearow (already evolved!), and drifloon are available prior to the 3rd Gym, while murkrow spawns a little after it. If that's too late for your taste, then swoobat and vivillon are bot obtainable before the first Gym and provide a tricky path to power, while the pidove line (attained before the 2nd Gym) gives a more straightforward path (and is required for a sidequest in the midgame anyway). You've got a few options you can work with here!

Thanks

What is Good Pokemon to use for reborn 

First things first, It's worth pointing out that a pokemon is just as likely to get their Hidden Ability as they are to get their normal ones. This naturally makes a number of pokemon better options here than they would be in a normal playthrough.

For example, blaziken is widely considered to be the single best Starter Pokemon in the game... if it has Speed Boost. Fire/Fighting is a powerful Type combination that wrecks most of the game, and Speed Boost goes a long way towards mitigating the drawbacks of blaziken's strongest moves. Aside from that, most of the Fire Starters are powerful picks, as are totodile and especially froakie (note that Battle Bond doesn't exist in this game, so you should use Protean if this one's your Starter).

Krikitune reportedly shreds through the early game, thanks to the vicious nature of Technician-boosted Fury Cutter. It does fall off midway through the game, but you'll have far more options by that point in the game anyway.

Don't sleep on klink in this game- both of its relevant signature attacks are highly improved in what is arguably the most common Field Effect in this game, allowing it to grind much of the storyline to dust on its own. Even without the boosts, Shift Gear and Gear Grind are still pretty decent attacks, and access to Mirror Shot allows it to hold its own in one of the most disliked Gym Leaders in the early-mid game to boot.

Nature Power is basically the ultimate coverage move in this game, and conveniently the second Gym's TM reward, so pokemon that can learn it are pretty helpful to have on hand. Most of these pokemon are either Grass Type, or Fairies that make people wonder why they aren't Grass Types, so available pool of pokemon is surprisingly small here. Of particular note are budew (obtainable before the first Gym), which becomes a decently fast Special Sweeper and has 3 guaranteed IVs of 31, and lotad, which doubles as a Bulky Water and learns Nature Power by level up, giving you access to the move before the second Gym even opens for you.

One last tip: every Gym has a pokemon of a Type it's weak against available nearby. If your current team can't break through a particular enemy, be sure to check out the nearby area for a potential counter!