Based on the SNES version...
I'd rank Hero, Bianca, Borongo, Flora, Your Son, Your Daughter, Sancho, Pipin, Bella, Papas, Harry, Mildrath, Gema, Jahmi/Jami/Centaursus/Centares, Ivol, Gonz, Buorn, Ramada. Also, Esturk FW >_>
Anyway...I think at a bare minimum, Hero, Bianca, Borongo, Your Son, Your Daughter, Sancho. Obviously, that is the smallest ranking, and minimizes it significantly.
For permanent PCs...Pipin is very minimal and boring, so you could skip him. Flora...you don't get a chance to see unless you pick her, which is 50% (Bianca you get to probably at least level 20-25 or so, so you have an idea of what she does).
As for temps, Harry and Papas are strong ranks, plot-important, present, etc. Bella is...relatively ok to keep unranked, I think, but she's still memorable enough and has plot importance.
Bosses...Mildrath is the final lord of darkness, but does nothing until the end (unlike Necrosaro). Gema *SPOILERS* Kills Papas, so he has some plot relevance. Ivol is the leader of the religious fanatics supporting Mildrath. Jahmi/Jami/Centaursus/Centares *SPOILERS* turns Hero and chosen wife to stone, and helps with what Gema did. Gonz helped Gema...and that's about it. Buorn is a major boss with some plot, particularly with Flora. Ramada is a high priest, but nothing else really. At a minimum...Gema, Ivol, Mildrath, probably?
Don't support monsters myself, or at least, ranking just one monster over the others. It doesn't help I didn't get a Slime Knight until late generation 2, and already had King Healers, Curers, etc. as back-up healers. Rank Slime!?