First Time Play Through Advice?

Hello! I'm playing Pokemon Odyssey right now and just beat the First Stratum. I'm kinda overwhelmed by the number of alternative forms and was wondering how viable most mons are?

Is there any reason to keep plusle and minun? I notice they get some sort of bond form or something later on but dont know much about it. I caught a shiny lotad pretty early on with decent ivs so I might keep him on my team already.