Hi /vp/, Pokemon noob here

Haven't played since genwun and hoping you guys could answer some quick questions for me, can't find a clear explanation through google.

I'm looking into making a competitive team for online battling. I was hoping to use poliwhirl and haunter since they're too of my favorite pokes, but I've got a poliwhirl trained up currently and he is getting annihilated every match. Does not evolving a pokemon hold back its stat potential? Will i need to evolve poliwhirl and haunter, or is there a way I can make them competitively viable?

My poliwhirl's moveset is: belly drum, waterfall, earthquake, and rock smash. He is adamant nature, super trained for attk and spd. Anything I can change?

Secondly, I noticed that online matches scale everyone to 50. Is there any benefit gained by using higher pokemon other than access to higher level moves, or are there stats identical to their level 50 self?

Thanks. Hope these questions aren't too obvious, I've been away from pokemon for long that most of this stuff is completely new to me.