The only think she would have in common with our favorite fish Paletuna would be the recovery, a teleport move, I guess. There's many things Sakurai can use to make her different: claws, uppercuts, snakes... Even if Medusa uses her staff, it wouldn't look like some Twirling Baton moves.
Ignorant non-KI fans and ignorant Smash players, yes. Same applies to Fire Emblem indeed.
Lucina and Dark Pit would probably be not hated if they had no slots on the character select screen, yet retain their differences. Do people complain of the number of Mario characters because of Dr Mario or the Koopalings? Dr Mario of course!
4+1 FE characters is fine. We have 4 FE eras like that, and as a DLC I consider more Roy like a fanservice bonus for Melee players. So to me, it feels like 3+2.
3+1 KI characters is fine too. Except Magnus, every KI character have something interesting to offer. Especially Miss Sideboob
Well, beside summoning some Underworld Army monsters, I don't really see what she could borrow. I can't imagine her with the Eggplant Wizard's powers.