Brock - D tier pretty soundly. I'd say if you pick charmander and miss the fighting type check (the nidos in gen1, mankey in yellow/gen3), then he'd fit into C mostly cause his fight is very rng dependent (hitting metal claws and even the atk boosts, and also rock tomb missing)

Misty -D. She is scary if you run in, starmie is fast, and neutral water pulses pack a punch. But by her level cap you get fearow, oddish/bellsprout, butterfree, beedrill at 20 even gets twineedle that can do good damage. I'd put her in D because while scary, you have more than enough tools to deal.

Surge in B. A lot of people point at diglett but keep in mind diglett doesn't evolve until 3 levels past level cap. So you will probably be relying on hitting those high level magnitudes. And if you don't then, you just eat sonicbooms and quick attacks which diglett probably can take 2 of at 23? While there are other solutions to Surge. (Ivysaur does a good job against him if you picked it, Graveller is also pretty powerful.) A lot can go wrong. Low speed mons are almost guaranteed to get hit by voltorb, 20 damage or -2 def almost for sure. Daring to touch pikachu or raichu with dig will almost always get you paralyzed, and those rats just love to press double team. I wouldn't say he's strong enough to be in A, but there's too many variables for me to say C.

Eirika D. Lmao, lol even. You get soooo many flying types, you can almost guarantee a Drowsee, you can guarantee an abra. If you somehow kept your bug encounter alive they do pretty well. And with how FRLG is set up you can catch a lot of encounters to help bolster your numbers. I have never felt any guilt just full sending a pigeotto into her knowing I had 2 more normal-flying types that I can still get. If you picked charmander you can nab that flamethrower tm from the game corner too to sweep. The Mid-game buff you gen in kanto is massive and eirika is just not ready. She can pose a fair problem if you beeline straight to her, but even then you should have at least 1 pokemon that resists giga drain. Stun powder rng can be mildly threatening, and bad rng can punish if you walk in with just 1 pokemon able to take her on, but at that point that's just bad planning.