@Yacobe
Welcome to PRO! There's several pokemon that are disgustingly good in PRO's PvE, Gengar being one of them. I generally recommend you look out for pokemon whose rarity are common, whose final evolution is a great PvP pokemon (which in the majority of cases will translate well into PvE) and that is easy to train. Some examples may be Machop to Machamp, Geodude to Golem (more for PvE than PvP), Magnemite to Magnezone and Slowpoke to Slowbro. You aren't really focusing on PvE lategame for now (like bosses) but many good PvE story sweepers can also be used for specific bosses and quests, so they'll be useful.
-Are there any Kanto pokemon I can sell for money?
This heavily depends on PvP, basically. It is hard to sell pokemon that are exclusively good in PvE (such as Minun, Plusle or Dustox who are amazing in PvE) but not good at all in PvP. If you are capable of hunting for Synchronise Natu in Route 22, I seriously suggest you try your luck with Timid Gastly in Vermillion Graveyard (though it requires 31 speed IV to be sellable at a good price), Jolly Magikarp in Viridian City or Timid Magnemite in Power Plant. It is a bit hard to hunt PvP pokemon at the beginning of the game but it is entirely possible.
-Can I get the other starters in Kanto?
Yes. Squirtle is huntable in Vermillion City by Surfing. Charmander is available in Cinnabar Mansion. Bulbasaur is available in Kanto's Safari.
-Should I try to complete my dex?
Yes. It gives you access to additional boss fights, quests and specific areas. Also access to legendary pokemon.
-Can/should I fight bosses?
If you are a newbie: yes, after you beat the Kanto Elite 4, grab a few level 100 and challenge some bosses in EASY mode. By the time you have made a boss team you will have fought bosses a few times and be familiarised with them to fight them in harder difficulties. Their cooldown to be refought is 12 days. Take your time! There's no rush. They give a lot of money and useful items.
-Is there any form of breeding?
No, as breeding destroys the economy, it is not enabled in PRO.
-Can I bring pokemon between regions?
No but yes. Once you start a new region you'll have to beat all gym badges in that region with pokemon you find there. Once you beat the 8 badges you'll be able to use your old pokemon again.
Hope it helps!
Lonely Pikachu for story is fine. Pikachu is a mixed attacker and with Light Ball it hits really hard during story. It mainly uses special attacks but it can also use some physical ones like Dig, Volt Tackle and Iron Tail.